Added automatic discovery mechanism for partials

[BennettJames]

Alright, managed to piece together something that is pretty tidy compared to what I originally outlined in #9 and seems to work fine in browser & server-side.

Basic explanation of what goes on here: before translating a template, it parses the template into an AST. This syntax tree is then recursively searched for all partials, which are then added as a dependency to the currently-parsed template. Each dependency is required and registered as a partial, and the precompiled template is exported (as before). The exact same process is applied to each partial.

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A few other notes:

• The register-partials-as-dependencies system this uses can result in a partial being registered multiple times. I don't think this will ever be an issue, as re-registering the same template twice doesn't look like has any effect on Handlebars.
• The way it travels the AST is really rough. It looks at every single value in every object in the returned AST object. I couldn't find much by ways of official documentation on how to deal with partial discovery, and this seems to work "well enough".
• if you think this looks good, I should probably add some documentation to the readme to clarify how partials are handled.