Evaluate bats as a replacement for shunit2

[calebcartwright]

The git hook script files are one of the three core pillars that make rusty-hook work, and as such they are unit tested pretty thoroughly. The tests are currently leveraging shunit2 which is certainly sufficient, though I think it may be worth investigating whether we could leverage bats instead.

This is a relatively low priority but opening to keep track

[calebcartwright]

Have been using Bats in other projects and loving it. It will require the incorporation of submodules to pull in things like the assertion library, and I've not had a chance to play with Bats on Windows, but definitely want to pursue this