Improve usage/help text of submit command.

[kytrinyx]

This twitter conversation is pretty revealing: https://twitter.com/codefancier/status/738070299875246080

[kytrinyx]

The current output is:

$ exercism help submit
NAME:
   exercism submit - Submits a new iteration to a problem on exercism.io.

USAGE:
   exercism submit [command options] [arguments...]

OPTIONS:
   --test       allow submission of test files
   --comment value  includes a comment with the submission

The USAGE should be

exercism submit [command options] file1 [file2, etc...]

or something like that.