'TextHelper#highlight' now accepts a block to highlight the matched words.

The helper will yield each matched word, and you can use this instead of the
':highlighter' option for more complex replacing logic:

  highlight('My email is me@work.com', EMAIL_REGEXP) { |m| mail_to(m) }
  # => 'My email is <a href="mailto:me@work.com">me@work.com</a>'