Uses correct argument character for BC on non-windows platforms

Based on #26

Author: droyad

src/Assent/Reporters/DiffPrograms/BeyondCompareDiffProgram.cs

@@ -42,7 +42,8 @@ public BeyondCompareDiffProgram(IReadOnlyList<string> searchPaths)
         protected override string CreateProcessStartArgs(string receivedFile, string approvedFile)
         {
             var defaultArgs = base.CreateProcessStartArgs(receivedFile, approvedFile);
-            return $"{defaultArgs} /solo";
+            var argChar = DiffReporter.IsWindows ? "/" : "-";
+            return $"{defaultArgs} {argChar}solo" ;
         }
     }
 }