Add support for Hamlet Message interpolation.

e.g. _{YesodMessage}
Support for @?{..} variable interpolation.

This allows embedding of query string parameters which can
be useful for creating paginated responses.
Font-lock variable interplolation inside various html tags.

Fixes: <img src=#{var}>
Update README with new interpolation syntax.
Merge pull request #3 from creichert/master

Add support for Hamlet Message interpolation.

fixes #2