pass extra context settings to completion

[davidism]

The completion system created a context that ignored any extra context settings passed in like cli(obj={...}, color=False). These are now passed to the completion system, so they're reflected in the context that is built for completion.

fixes #942

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.