Add support for whitespace control mustaches

See http://handlebarsjs.com/block_helpers.html#whitespace-control

Ported from https://github.com/JetBrains/intellij-plugins/commit/f2d78a1851a6ce5315279cc146a82cd34fd0f51f#diff-b644d7b7e647851e177e6d311c1a185d