Removed the dance with selected folders.

  We can get the current folder name via "current_maildir()" directly:

    http://lumail.org/lua/current_maildir.html
  Simplify the favourites function.

  We can use a table to allow an arbitrary number of folders
 to be opened.
  TODO: Prompt for y/n - now completed.

  Prior to sending the mail via compose() we prompt for confirmation.
 If the user enters [nN] then we don't sent the mail.
  Updated comment, since we do prompt for y/n now.
  select_maildir() works.

  This allows the list of maildirs to be jumped to the given
 folder, if present.

  This closes issue #17.
  Allow lua-functions to be called on keypresses.

  Until now all keyhandling was carried out via the keymap
 table.  Now there is a lua-defined function "on_key" defined.
 If that function exists it will be called on every keystroke
 *BEFORE* our table-lookup.

  If the on_key function returns >0 then further processing
 will be stopped.  Otherwise we'll revert back to the table-lookup.
  Updated and mentioned `make install`.
  Added "install" target.
  Added helper function for creating a new maildir entry.

  This will be given a path and will create a "random" filename.

  Part of issue #4.
  Write a copy of sent-messages to disk.

  We now archive the message the user composes to disk, via
 the sent_mail path.

  Closes #4.
  Refresh the prompt after each keypress in prompt_yn().

  This looks neater.
  Writing outgoing mail works correctly now.

 Previously there was a minor error which lead to NULL bytes
 being output at the end of our reading attempts.
  New release 0.06.