Implement lean packaging of tilt

Gemfile:

    # register with all available template engines, default option
    gem 'tilt'

Gemfile:

    # register with no loaded templates, expert option
    gem 'tilt', :require => 'tilt/lean'

boot.rb:

    # require only needed template engines, no bloating, fast load
    require 'tilt/haml'
    require 'tilt/sass'