Fix bug with directives like #title leaking out of included regions

Fix bug with directives like #title leaking out of included regions

* lisp/muse-publish.el (muse-publish-cite-tag): Don't let-bind
  muse-publishing-directives, because there is no need to do so.  Fix
  code indentation.
  (muse-publish-markup-attribute): Avoid multiple evaluation of attrs
  argument, in case it is list value instead of a symbol.  Use sexp
  instead of form in the edebug-form-spec.
  (muse-publish-include-tag): Make a full copy of
  muse-publishing-directives, since it can be modified with setcdr.  This
  fixes a bug where #title and some other directives were leaking out of
  an included region.
  (muse-publish-mark-up-tag): Don't let-bind muse-publishing-directives,
  because there is no need to do so.