V0.5.0 - replace all occurrences of {{>body}}. See issue #467

Hi again everyone!
This is my first attempt at tackling issue #467.

I used global string.replace for the `{{>body}}` bit, seems to work fine.

Also added a couple of tests, on which I have a few questions...

* I was debating whether test/fixtures/templates was the right path for those layouts/templates. Maybe something like test/fixtures/layouts would be better?
* Actually, do I even need the files? Is there a better, preferred way (for tests) for injecting simple templates like that? Or just using files is fine?
* Was also debating what was the best place to put the new tests. I ended up using the existing `source_test.js` but was wondering if I should create a new file (like `layout_test.js` or something).

Please let me know what you think; this is a super cool project and I'm really glad to try and contribute. :+1: