CLI make colourization optional for cmd --help

[hjoliver]

This is a small change with no associated Issue.

Some of our command help pages are quite long, so I sometimes like to grep in them, or open them in an editor cylc play --help | vim - in which case color control codes in the output are annoying. This hack makes --color=never work with --help.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.py and conda-environment.yml. (No changes)
• Does not need tests (why?).
• Appropriate change log entry included.
• No documentation update required.

[oliver-sanders]

(recommend cylc help | less -r)

[oliver-sanders]

Looks fine.

[hjoliver]

This is a trivial change that can't impact anything important, so I'll take @oliver-sanders' "Looks fine" as approval.