feature separation of supports-colors

[indietyp]

Implements #88.

This separates the supports-colors feature into three different features:

• supports-colors
• std
• override

The override methods (and if_supports_color) are now enabled with the override feature.

This also "fixes" #82 by introducing a new Stream type, essentially the same as supports-colors, but needs to be implemented as a new type as it is exposed when the override feature is enabled. (a From implementation is provided).

This also makes all tests no-std compatible by introducing a new helper: assert_str.

(all permutations also have been tested using cargo hack test --feature-powerset)

[jam1garner]

Very sorry for letting this grow stale, I've been locked out of github for a while and only really been able to push commits. I'd really hate to ask you to redo the work to be compatible with #115 so I think I'll just close this and I'll try and see if I find the time to redo the work myself before pushing 4.0