Add --version command line option

I went with a case statement because `getopt` on BSD only supports single dash (short) options. I suggest moving the main runner code into a method that is called from the case statement similar to how [we have done it in fresh](https://github.com/freshshell/fresh/blob/4fe4036943ac30b6e5ecd6b41ed47240af668a53/bin/fresh#L593-L628) that way as more options get added the case statement will be easy to read. Let me know if you want anything changed.

[Diff without the whitespace noise](https://github.com/shpec/shpec/pull/29/files?w=0).

Closes #19.