Support whitespace control syntax from handlebars

see http://handlebarsjs.com/block_helpers.html at bottom section
„whitespace control“

Opening and closing curled braces allow an optional "~"-character in order to not mark this syntax as an error. 

Tried to write some unit tests for this, but couldn't make the project compile with the version of the master branch. Templates using this syntax pre compile successfully. 

Handlebars supports this feature since 1.1.0
https://github.com/wycats/handlebars.js/blob/master/release-notes.md#v110---november-3rd-2013