Unit tests

[carlosedp]

I'd love to have a couple of test cases to make sure the compiler is working.

Could you help out on this @RoadRunnr and @ebengt? I'm a little rusty on Erlang :)

[RoadRunnr]

I'm a bit of a loss of how a test for the plugin would work. I've checked a few of the other rebar3 plugins for inspiration, and almost none of them have tests.

[ebengt]

I have an idea about test, but that will have to wait until after work hours.

[ebengt]

I made a pull request with a test. #15

[carlosedp]

Fixed by #15