Improve auto-detection of color-capable terminals

Terminals such as 'screen-256color', 'rxvt-unicode-256color' etc.
were not automatically detected as color-capable.

https://github.com/CapnKernul/minitest-reporters/pull/60#issuecomment-15015863
Document how to obtain color output from DefaultReporter
Merge branch 'patch-1' of https://github.com/aspiers/minitest-reporters into develop
Fix some minor formatting issues.
Merge branch 'develop' of github.com:CapnKernul/minitest-reporters into develop
Merge branch 'master' of https://github.com/aspiers/minitest-reporters into develop