Write tests for the "yakshave new" task

[johnwalker]

I'm not very familiar with the testing environment in Clojurescript or nodejs. It'd probably be easier to test sooner than later.

Ideally, I'd like to have some tests that can be run to ensure that the right files are being written on supported platforms. However, I don't think that would be a lot of fun, and probably not as important as getting some unit tests running for the pure functions. This will likely require some refactoring.