Allow for more “sources” when creating default values

When setting default values the “sources” options are limited to “current post” and “url query variable.”
Two other very useful sources would be:
1. Another form field. For example, I’m editing a CCT with a related single_post. I can set a hidden variable to that “cct_single_post_id” then, use that hidden variable to set defaults for taxonomy and terms fields that relate to the single post id. As of now, I have to pass both the cct_id and the single_post_id in through query vars – and that’s messy.

2. A shortcode. This explains itself, we could set it from any number of sources with custom coding.