It would also have been valid to get rid of the attr_reader, but I like
being able to reach inside KeywordMute::Matcher without resorting to
instance_variable_get tomfoolery.
A matcher object that builds a match from KeywordMute data and runs it
over text is, in my view, one of the easier ways to write examples for
this sort of thing.
Gist of the proposed keyword mute implementation:
Keyword mutes are represented server-side as one keyword per record.
For each account, there exists a keyword regex that is generated as one
big alternation of all keywords. This regex is cached (in Redis, I
guess) so we can quickly get it when filtering in FeedManager.
