make-it-so.el: improve customization

* make-it-so.el (mis-make-command): New custom.
  (mis-make-key): New custom.
  (mis-config-default): New function.
  (mis-make-it-so): Update doc.
  (mis-abort): Update doc.
  (mis-finalize): Update doc.
  (mis-replace): Update doc.
  (mis-save-and-compile): New function.