Support double quoted heredocs

[netzpirat]

The single quoted heredoc is parsed fine, but there is a second notation with double quoted heredocs that allows interpolation within the heredoc.

Example

$("""
  <div id='#{ @getID() }' class='tree'>
    <ul class='#{ if @sortable then 'sortable' else '' }'></ul>
  </div>
  """)