asimov add <path>: add an exclusion by hand

[django23]

Split out of the roadmap (#51).

Proposal: asimov add <path> excludes a path immediately and records it in the
persistent cache, so asimov list and #38 (prune) both know about it.

Lower value than the rest of the subcommand set, since tmutil addexclusion <path>
already works and is a single command — the gain is only cache consistency and not
having to remember the tool name. Filing it so the roadmap split is complete; happy to
close as unnecessary if nobody wants it.