Simplify mutt configuration.

I no longer use separate mailboxes for differente addresses, but rather rely on
alternates for this. Deleted old multi-account settings, and merged those that
matter into muttrc.
Add a FIXME to muttrc.
Make grep(1) ignore VCS directories by default.
Delete sublime plugins.

Given that I haven't used sublime as my main editor (or, rather, at all), in a
long while, it's pointless to keep all those plugins as submodules (and they
get in the way when running submodule operations).
Highlight line breaks in markdown.