Make Sinatra specs work with both ERB and Erubis
Form objects now support a :hidden_tags option for automatically adding hidden tags

Previously, it wasn't possible to automatically add hidden form
tags based on the content of the form attributes.  At least, not
easily in a generic way.  By providing a proc that is called
with the form tag object itself, it's easy to conditionally add
such hidden tags.
CSRF token tags are now automatically added to forms in Rails and Sinatra if using rack_csrf (Fixes #5)

This automatically adds CSRF tokens on Rails and Sinatra when using
rack_csrf, making it so that you don't have to add such tokens
manually.

It also offers an easy framework to automatically add other tags
on Rails or Sinatra, via:

  Forme::Rails.add_hidden_tag{|tag| ...}
  Forme::Sinatra.add_hidden_tag{|tag| ...}