phabricator: add --blocker argument to phabsend to specify blocking reviewers The way to signal to Conduit that a reviewer is considered blocking is just to wrap their PHID in "blocking()" when including it in the list of PHIDs passed to `reviewers.add`. arc doesn't have a --blocker, instead one is supposed to append a '!' to the end of reviewer names (I think reviewers are usually added in an editor rather than the command line, where '!'s can be more hazardous). moz-phab (Mozilla's arcanist wrapper) does have a --blocker argument, and being explicit like this is also more discoverable. Even `arc diff`'s help doesn't seem to mention the reviewer! syntax. Differential Revision: https://phab.mercurial-scm.org/D6512