Allow "mhng-show" to change the current sequence number

This is necessary to make mhng-show actually work... :)
Use RFC-formatted addresses for all of mhng-show

I'd accidentally only formatted the From/To addresses this way, which
was straight-up wrong.
Implement "mhng-next" and "mhng-prev"

There isn't a whole lot to say here (I guess aside from the fact that
they're currently a whole lot cleaner than the old
implementation... :)).

This allows me to actually move around my mailbox... :)