recipes/cue/split/Makefile: generalize
recipes/flac/to-mp3/Makefile: works for multiple files
recipes/cue/split/Makefile: provide replaces output
make-it-so.el: update interface

* make-it-so.el (mis-source-files): New variable.
  (mis-target-files): New variable.
  (mis-make-it-so): Sets `mis-source-files', checks if they all have the
  same extension.
  (mis-abort): Simplify logic.
  (mis-finalize): Use "provide" file instead of "make provide".
  (mis-directory-files): Update doc.
  (mis-all-equal): New function.
  (mis-action): Improve.
  (mis-delete-file): New function.
  (mis-rename-quote): New function.