Add unit testing
Add unit testing

I added a bats script to test chext.  The script creates a hidden directory
called .chext and works within this directory, where it creates and moves
files.

Currently there are nine tests, which test the following:
- Usage message
- Basic functionality
- Options
- Error handling (non-existent files, etc.)

joe-antognini/chext,lafrenierejm/chext