Move colored diff output to feature flag

[repi]

This allows one to use this crate without bringing in the pretty big colored crate dependency and for when colored text diffs are not important.

The new color feature flag is enabled by default so no change in the default behavior

[lipanski]

fair enough. please add a line about how to use this feature flag to the documentation inside src/lib.rs, ideally somewhere around this line https://github.com/lipanski/mockito/blob/master/src/lib.rs#L138

thanks for your contribution 🥇

[repi]

Ok there!

[lipanski]

thanks again. released as 0.21.0 🚀