remove: implement a --cache option/behavior?

[jorgeorpinel]

Context: https://discordapp.com/channels/485586884165107732/563406153334128681/678885051815165953

This option would also remove the DVC-file outputs from cache (not just from workspace).

Use case: freeing up space used by intermediate results once a final result/modes is stable.

It would also make dvc remove more useful for single-output stages.

Similar to https://git-annex.branchable.com/git-annex-drop/

[efiop]

Will be solved after gc, because it is directly related to it #2325

[efiop]

Closing as stale.