Filter taxonomy_checkbox element values

[hardlyneutral]

I've added the ability to pass an option to taxonomy_checkbox that allows the checkbox element's value to be set to either the slug (which is the current default) or the term_id.

I need this functionality for a site that I'm working on, but I'm guessing this could be abstracted with a bit of work to be an option for all element values.

Thoughts?

[joshbetz]

I was thinking this would literally be a filter. I've added some more details to the issue (#98).