Add :force_encoding support to WebsiteAgent.
Ignore force_encoding if empty.
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.
make validate methods as an private method for avoiding to use it as an instance methods