runcli: rm unused 'color' and '**extra' params

Angelos Evripiotisrequested to merge
aevri/unused_color into master

The only customer for 'invoke' is the 'run' method on the same class. Make it private and simplify it's interface.

Avoid disappointment by removing unused 'color' and 'extra' parameters from 'invoke'.

These appear to have been here to mirror the interface of click.testing.CliRunner.invoke(), which this class replaced.

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