Allow force-enabling or force-disabling colorized output

[aneeshusa]

click allows emitted colored text via click.style, which is used by pip-compile to emit colorized e.g. comments to stderr. click uses auto-detection to only enable colors when writing to a TTY.

At Lyft, we operate pip-compile as a service: pip-compile invocations are forwarded to a set of remote machines, which leverage a shared cache for much faster compiles. However, they run pip-compile as a subprocess without a TTY attached meaning the output we stream back to the user is not colorized.

Therefore, add a --color and matching --no-color argument to force-enable as well as force-disable color (the default remains click's auto-detection). This could also be used for e.g. running pip-compile in CI.

AFAIK pip-sync does not emit colored output, so I did not add the --color/--no-color options there.

Contributor checklist

• [x] Included tests for the changes.
• [x] PR title is short, clear, and ready to be included in the user-facing changelog.

Maintainer checklist

• [x] Verified one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• [ ] Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[aneeshusa]

Thank you for taking a look @chrysle!