Add golden file tests

[micahhahn]

This PR adds a new primitive equalToFile to the Expect module. It also adds a prettyPrintSingle method

This primitive allows us to write golden file / snapshot tests in Elm. These are particularly useful at capturing the current outputs of a program and then failing if the outputs change.

NoRedInk in particular has an interest in snapshotting the HTML at various points in elm-tests to a file. I've also added a prettyPrintSingle function to the query module which exposes the internal query pretty print function. I think this will be useful generally for testing to be able to capture focus of a query in a complicated test (especially with elm-program-test). NRI would also see value in dumping the entire HTML of a page to a golden file so that we can do visual diffs of our pages in their various states.

Note that the code its current state is very rough - the kernel code added presumes that the tests are being run in the context of a node environment. That's always true for node-test-runner but AFAIK not always true for elm-test-rs. I'm very open to any ideas that would make this more robust.

I'm also aware that doing the IO direclty inside of the elm-explorations/test probably prevents the test runners from doing proper watching on these golden files. Definitely open to ideas on approaches that would let the actual file IO be handled by the runner itself.