Option to force colored output

[giner]

Following #246 it would be nice to have an option to force colored output when needed even when it is piped. This would be useful in scripts, e.g. output=$(spruce diff --colored ...) or with less, e.g. spruce diff --colored ... | less -R.

[phlummox]

Seconded :)

It seems to be reasonably standard to call such a parameter "--color", and let it take the options "always" (coloured), "never" (not coloured), and "auto" (coloured if a tty is detected). For instance, GNU grep, GNU ls, git log, shellcheck, and many other common tools use this naming scheme. It may therefore be worth doing the same, to benefit from users' likely familiarity with those tools.

[geofffranks]

Feel free to submit a PR for this @phlummox, it is https://hacktoberfest.digitalocean.com after all!