Allow data to be specified from a json file

Pretty much thats it.

Use `options: { data: 'path/to/json' }` to specify a json file as data for the template.

I've added a test but your test sling overwrites previous tests so you're never actually testing earlier tests (unless I've really misread it).  I think the partials target overwrites the output of the default_options target, so the default_options test is actually testing against the partials target.  The same happens with my test, not sure how you want to handle that so I just followed convention with the tests.