Some fixes to 'text' mode:

* Fix weird crash when printf characters appear in a line (I got a fatal
error from glibc "*** %n in writable segment detected ***" when including
a line which incidentally included "%n").
* Don't clobber the status line.
Merge branch 'master' of github.com:jugglerchris/lumail into expose_maildir
Merge pull request #207 from jugglerchris/master

Some fixes to 'text' mode:
  Reset text-position when updating text.

  This updates #210.
  Allow text-mode to get/set the line offset.

  This closes #210.
Merge branch 'master' into expose_maildir
  Update the maildirs when changing mail_prefix.

  This updates #209.

  (Probably closes it too, but there does seem to be some confusion..)
  Don't update maildirs unnecessarily.

  This closes #209.
Merge remote-tracking branch 'origin/master' into expose_maildir
Fix a warning (unused lua_tostring).
Export maildir::matches_filter() to Lua.