Allowed for template-lookup callbacks; fixed indentation

Hello!

I added a mechanism to provide a callback function which can load partials by name from some external cache. This requirement came about while I was working on a project which stores its templates in the database (and cached in memory) rather than on the disk, but also needs to load templates dynamically.

The additions are on lines 46-63, and 147-150 – it should be pretty intuitive. There is also test coverage, which hooks into the existing test fixtures.

And yes, I passive-agressively reformatted the code to be inline with idiomatic go standards. I make no apology for this. Sorry, but that's the way it is.

Thanks for this package, it's pretty boss!

PMF