Add regexp matching support to EventFormattingAgent.

A new setting key `matchers` is added to define regular expression
matching against contents of events and expand the match data for use
in `instructions` setting.
Add specs for :matchers validation.
Adjust the style to the project's taste after a review.
Add a missing assertion.
Merge pull request #214 from knu/event_formatting_agent-regexp

Add regexp matching support to EventFormattingAgent.