Proposal: Switch formatting to reformatter.el

[joachimschmidt557]

See #39 for the corresponding issue.

This changes some of the weird behavior which arises from using revert-buffer after formatting such as #49.

Of course, this massively changes formatting behavior for users used to the formatting functionality. For example, formatting on save is now handled by a new minor mode instead of a defcustom and a toggle function. This also adds a dependency on the reformatter package in MELPA.