Adds option for global fallback

Hello!

Thanks for this module, it's exactly what I was looking for.

I did run into one issue when using it. Having `require('handlebars')` in the file output by default breaks my browserify build, since I have Handlebars loaded in an external file. This is my stab at fixing the problem, what do you think?

If you think it's a valid or worthwhile addition, I can update the documentation and unit tests.