Add the option to sort maildirs in Lua.

There is now a Lua function "sort_maildirs", which if
present is passed a table of maildirs to sort and
return.
Add simple maildir sorting example.
Merge pull request #212 from jugglerchris/master

Allow sorting maildir list in Lua.
Fix a crash with prompt strings containing '%'.

Replace a couple of 'printw(str)' with 'printw("%s", str)'.  I otherwise
got a crash on eg prompt_chars("blah%n", "abc"):

  ** %n in writable segment detected ***

  Program received signal SIGABRT, Aborted.
Merge pull request #213 from jugglerchris/master

Fix a crash with prompt strings containing '%'.
Change current_maildir() to return a CMaildir.
Merge pull request #215 from jugglerchris/master

Change current_maildir() to return a CMaildir.
  Updated such that 'make test' passes.
Merge branch 'master' of github.com:skx/lumail