Change program usage to be more verbose.

The usage output was changed to include a description of what the
program does, along with some examples for running it.  This matches
how most of the standard command line tools behave (for instance,
`cat`).

This fixes issue 19.
https://github.com/fujimura/hi/issues/19
Merge pull request #20 from cdepillabout/issue-19-usage-more-verbose

Change program usage to be more verbose.