Add common test supports

Hey,

This add common tests support, the syntax is like this:

`rebar3 ct`

You can pass any option on to ct using the `=` syntax.

`rebar ct suite="suite_a, suite_b"`

Long options need to be quoted. This is probably not ideal. Work in progress.

`short_desc` should be like "Run Common Test suites". "Run common tests" sounds like something generic/vague if you don't know Common Test. Not that you would be running `rebar ct` if you didn't... meh, I don't know.