Add BATS as submodule and initial CLI tests

Executed this command to add BATS as a submodule:

    $ git submodule add https://github.com/sstephenson/bats.git tests/bats

Borrowed test_helper from nodenv:

    nodenv:test_helper: b9d86f13cb1a6fed45e881c1a92f46e433b80047