loaded as worker detection and support

[seacgroup]

starting at line 567, detect if running as worker, if so add message listener

[seacgroup]

well yeah going to fail in Travis cl build, I suppose it can be changed to:
if ( typeof self === 'object' && self.WorkerGlobalScope && self instanceof WorkerGlobalScope ) {

so what platforms are we talking about that don't support self?"

[mjackson]

My preference would be to leave this as a 3rd party add-on to the core mustache.js library in a separate repo.