Activate by require-ing 'minitest/documentation'
Add generic test case
Remove color formatter

It will be available at https://github.com/teoljungberg/minitest-color
Refactor `io` to generic test_case class
Prepp for release

* Activate by require'ing `minitest/documentation` aswell as passing `--color`
* Color formatter have moved to https://github.com/teoljungberg/minitest-color