feat(from_usage): adds ability to add value names or num of vals in usage string

Allows new usage strings with value names or number of values. If the
names are consecutive, they are counted to represent the number of
values (if they all have the same name), or if their names are different
they are used as value names.

Closes #98

Author: kbknapp

src/args/arg.rs

@@ -1,4 +1,5 @@
 use std::iter::IntoIterator;
+use std::collections::HashSet;
 
 use usageparser::{UsageParser, UsageToken};
 
@@ -79,7 +80,7 @@ pub struct Arg<'n, 'l, 'h, 'g, 'p, 'r> {
     #[doc(hidden)]
     pub group: Option<&'g str>,
     #[doc(hidden)]
-    pub val_names: Option<Vec<&'p str>>,
+    pub val_names: Option<Vec<&'n str>>,
     #[doc(hidden)]
     pub num_vals: Option<u8>,
     #[doc(hidden)]
@@ -229,34 +230,57 @@ impl<'n, 'l, 'h, 'g, 'p, 'r> Arg<'n, 'l, 'h, 'g, 'p, 'r> {
          let mut required = false;
          let mut takes_value = false;
          let mut multiple = false;
+         let mut num_names = 1;
+         let mut name_first = false;
+         let mut consec_names = false;
+         let mut val_names = HashSet::new();
 
         let parser = UsageParser::with_usage(u);
         for_match!{ parser,
             UsageToken::Name(n, req) => {
-                if name.is_none() {
+                if consec_names {
+                    num_names += 1;
+                }
+                let mut use_req = false;
+                let mut use_name = false;
+                if name.is_none() && long.is_none() && short.is_none() {
+                    name_first = true;
+                    use_name = true;
+                    use_req = true;
+                } else if let Some(l) = long {
+                    if l == name.unwrap_or("") {
+                        if !name_first {
+                            use_name = true;
+                            use_req = true;
+                        }
+                    }
+                } else {
+                    // starting with short
+                    if !name_first {
+                        use_name = true;
+                        use_req = true;
+                    }
+                }
+                if use_name && !consec_names {
                     name = Some(n);
+                }
+                if use_req && !consec_names {
                     if let Some(r) = req {
                         required = r;
                     }
-                } else if let Some(l) = long {
-                    if l == name.unwrap() {
-                        if let Some(r) = req {
-                            required = r;
-                        }
-                        name = Some(n);
-                    } else if n != l {
-                        name = Some(n);
-                    }
-
                 }
                 if short.is_some() || long.is_some() {
+                    val_names.insert(n);
                     takes_value = true;
                 }
+                consec_names = true;
             },
             UsageToken::Short(s)     => {
+                consec_names = false;
                 short = Some(s);
             },
             UsageToken::Long(l)      => {
+                consec_names = false;
                 long = Some(l);
                 if name.is_none() {
                     name = Some(l);
@@ -270,6 +294,15 @@ impl<'n, 'l, 'h, 'g, 'p, 'r> Arg<'n, 'l, 'h, 'g, 'p, 'r> {
             }
         }
 
+        if let Some(l) = long {
+            val_names.remove(l);
+            if val_names.len() > 1 {
+                if name.unwrap() != l && !name_first {
+                    name = Some(l);
+                }
+            }
+        }
+
         Arg {
             name: name.unwrap(),
             short: short,
@@ -282,8 +315,8 @@ impl<'n, 'l, 'h, 'g, 'p, 'r> Arg<'n, 'l, 'h, 'g, 'p, 'r> {
             possible_vals: None,
             blacklist: None,
             requires: None,
-            num_vals: None,
-            val_names: None,
+            num_vals: if num_names > 1 { Some(num_names) } else { None },
+            val_names: if val_names.len() > 1 {Some(val_names.iter().map(|s| *s).collect::<Vec<_>>())}else{None},
             max_vals: None,
             min_vals: None,
             group: None,
@@ -765,8 +798,8 @@ impl<'n, 'l, 'h, 'g, 'p, 'r> Arg<'n, 'l, 'h, 'g, 'p, 'r> {
     /// # ).get_matches();
     pub fn value_names<T, I>(mut self, names: I)
                                  -> Arg<'n, 'l, 'h, 'g, 'p, 'r>
-                                 where T: AsRef<str> + 'p,
-                                       I: IntoIterator<Item=&'p T> {
+                                 where T: AsRef<str> + 'n,
+                                       I: IntoIterator<Item=&'n T> {
         if let Some(ref mut vec) = self.val_names {
             names.into_iter().map(|s| vec.push(s.as_ref())).collect::<Vec<_>>();
         } else {