Support loading design documents from a JavaScript file

Hello,

I'm looking to move off of `grunt-couchapp` and to use this grunt task instead.  One of the things I am missing is the ability to unit test my view functions before uploading the document (using a javascript test framework like jasmine).

This pull request adds support for loading a document from a javascript (`.js`) file, which should make this task compatible with `grunt-couchapp` documents, and allow myself (and others) to transition to this grunt task with very little effort.

I've also added a test for the existing behavior of loading a design document from a `.json` file.