Add support for customizable filenames

Far from perfect at the moment, but it works. Not sure how we're going to deal with the `-` in cropped. If we're setting it in the template there will be a empty `-` at the end of files that aren't cropped.

Fixes #35.
No need to save to disk. Just do like: https://github.com/sindresorhus/pageres/blob/7b8bda1cbf96432b39f9dd7aaebdb987cd378909/test/test.js#L87