worf.el (worf-copy-heading-id): copy :CUSTOM_ID:

* worf.el (worf-copy-heading-id): Copy :CUSTOM_ID: if it's available,
  otherwise copy/create a regular :ID:. Create :CUSTOM_ID: when arg
  isn't nil.
worf.el (worf-refile): add and bind to "r"

When called with prefix, refile to other file.