- Adds StructOpt argument parsing
  - Accepts `-` as a shorthand for standard input
  - Defaults to `-` if no path is given
  - Errors out via ExitFailure

For such a small amount of code, there's quite a lot happening here that I
don't understand yet.

Takes stdin and splits on whitespace (I'm guessing this only supports
UTF-8 at the moment).  Takes no flags, doesn't do anything fancy to
uniq-ify words, strip punctuation, etc.  Doesn't yet know how to open a
file.