Added --eval command line flag.

  This evaluates and exits.  Example:

      ./lumail -f ~/Maildir/ -r ./lumail.lua -e "dump_unread();"
  Removed debug message.

  This showed up when calling --eval.
  Updated to ignore generated files and clean them.
  Remove some more files on `make clean`.
  Added the sent_mail() primitive.

  This can now be used to complete the compose() function,
 storing a copy of the outgoing messages.

  Updates issue #4.
  Updated to use an unordered_map.

  This commit adds a hash-map for storing key=value settings,
 and updates the get_maildir_prefix/set_maildir_prefix functions
 to use it.

  This is part of the code required for issue #20.

  Rather than updating all the get/set_foo() functions we should
 remove them all and switch to using get_variable() and set_variable().

  Ideally we'd also expose the variables to lua.
  Expose our settings hash to lua.

  The new (undocumented) function "get_variables()" returns a
 table/dictionary of all known settings.

  This currently just has the maildir_prefix, but we'll move more
 to it, as part of issue #20.
  Don't abort if a setting is NULL.
  Added comments.
  Moved "from" + "sendmail_path" to using our hash-map.

  This removes the appropriate member-functions too.

  Part of issue #20.
  Updated to use our hash-map for more settings.

  This updates issue #20.

  Almost all suitable settings have been updated now.
 The next step is to drop the various get/set functions.

  e.g  set_mode( *mode ) -> set_variable( "global_mode", mode );
  Updated to use our generic variable handlers.

  This updates for issue #20, removing some hardwired functions
 and replacing them with `get_variable` + `set_variable`.
  Removed the get/set_index_format functions.

  These are replaced with calls to get_variable( "index_format").

  This is part of issue #20.
  Added simple helper function for string-variables.

  This simplifies our get/set routines.
  Updated comments.

  Some things are used now.
  Updated comments.
  Updated comments and indentation.
  Don't exit after --eval is used.

  We've added a new flag --exit.  This will cause lumail to exit
 after evaluating the string set with --eval.  If it isn't used then
 the eval-call will not terminate.
  Removed get_mode/set_mode.

  This means issue #20 is almost done.
  Removed get/set_maildir_prefix.

  This closes issue #20.