Ability to customize file extension instead of `.snap` (e.g. `.html`)

[gojanpaolo]

Hello, we currently have a bunch of tests comparing html files. We'd like to use snapshooter to compare those files since we're already using snapshooter on other projects (this way we can have a consistent workflow). It would be nice if the generated snapshot file extension can be changed to .html (or any other extension), for convenience and also a "correct" file extension. Thank you!

[relfman-cmg]

I would also like this to be available as well!