Centralize commenting functions into a single file, and declare
variables used by those functions in language support files. The
functions are `comment-selection` and `comment-line`, users should bind
those functions to the keys of their liking for easy access.
The modified files used to handle formatting code with the same code,
and a different command, the process is now centralized into a single
file. Language support files should set the `formatcmd` string to
whatever formatter is the most popular in their respective community,
eventually with default options.
