asimov rm <path>: remove an exclusion

[django23]

Split out of the roadmap (#51).

Removing a single exclusion means knowing the raw tmutil incantation:

tmutil removeexclusion /path/to/dir

Proposal: asimov rm <path> wraps it, and also drops the path from Asimov's
persistent cache — otherwise the next run silently re-excludes it, which is a confusing
outcome for anyone who just asked for it to be removed.

That cache interaction is the real reason this belongs in Asimov rather than a README
snippet. Related: #14 (path-scoped opt-outs) is the permanent version of the same
intent; rm is the one-off.