Add $browser_abbreviate_mailboxes.

Add $browser_abbreviate_mailboxes.

This allows '=' and '~' mailboxes shortcuts to be turned off in the
browser mailbox list.

The default $sort_browser value of "alpha" uses strcoll(3), which
ignores some punctuation.  When using multiple accounts with
dynamically changing values of $folder, this can lead to an
unintuitive sorting order.  (See issue #22 for an example.)

This problem came about because of commit 8328ce68, which enabled
mutt_pretty_mailbox() calls for IMAP and POP mailboxes too.  The
commit had a valid point about consistency and too-wide mailbox
listings, so I don't really want to revert it.

This option provides an alternative to completely enable/disable the
shortcuts for those who run into an issue.