Allow to define options to display colors

Related to #1458.
To explain the context of initial check for "possible" colors in 4.3.

Tons of project use colors=true, which make their configuration file unusable.
This check (in PHPunit) make them usable (without having to change, downstream, tons of config files)

So colors=true should means auto to keep 4.3 behavior.

If you allow colors=force (in configuration files), project will start to use it, and thus make configuration file unusable again.

Having --colors=force in command line options is fine, and project can use this option in their travis config file.