Rework KeywordMute interface to use a matcher object; spec out matcher. #164.

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.

app/lib/feed_manager.rb

@@ -138,7 +138,7 @@ class FeedManager
   end
 
   def filter_from_home?(status, receiver_id)
-    return true if KeywordMute.where(account_id: receiver_id).matches?(status.text)
+    return true if KeywordMute.matcher_for(receiver_id) =~ status.text
 
     return false if receiver_id == status.account_id
     return true  if status.reply? && (status.in_reply_to_id.nil? || status.in_reply_to_account_id.nil?)