re-organise mutt-wizard's files to not clash with mutt/neomutt/etc installed alongside

[jmtd]

As an existing (long time) mutt user I already have a byzantine collection of rc files that I rely on. I'm still curious, though, to see what mutt-wizard's opinionated configuration is like, so I want to give it a try. Unfortunately, trying to use it in the same account as any existing configuration seems to be a problem.

As a general principle I think it would be good if mutt-wizard isolated all of it's configuration (which I mean to include mutt/neomutt configuration generated/managed by mutt-wizard) from the normal paths that are used by all such programs.

To give an illustration, my normal alias for launching my work mail, neomutt -F path/to/work/muttrc, changed behaviour due to mutt-wizard writing ~/.config/mutt. Similarly, shipping /usr/bin/openfile seems like it could lead to trouble, and it could be in a mw-specific path such as under /usr/lib (which would also mean it wouldn't need a manpage).

This is a bit similar to #916 but I filed separately because I think the approach should be different than outlined in that issue.