Add unit testing via BATS

[erikvip]

I'm making some fixes / enhancements to optparse & created a basic test suite using BATS to ensure everything is working. Would be nice to get this all into master ...

Usage and general process notes are documented in CONTRIBUTORS.md.

It's real easy to use:

• Install bats (On Ubuntu it's in apt)
  • apt-get install bats
• From project root, run bats tests/

Two of the tests will fail, as they're testing for nk412/optparse#15 and nk412/optparse#17. Everything else should be passing.

I hope this will help spark some more development on optparse. It's a cool little utility, I like it.

If not, I'm gonna fork to erikvip/optparse2 and get all my fixes & the community fixes merged into one branch.

Thanks