add Handlebars docset

http://handlebarsjs.com

Been wanting this for a really long time, discovered it wasn't that hard to do it myself! 

Handlebars has over 7000 stars on GitHub which should be enough to deserve a home in Dash. This docset provides Guides for each file with Sections for each header and Functions for the API references. Each Guide is marked up with table of contents anchors for easy access.