Support custom taxonomy checklists

[Adam Winer]

Can't remember exactly why I didn't allow custom checklists for custom taxonomies. It doesn't seem that hard for most of the UI. But the major questions (which intersect each other) would be around:

• Where to store said checklists in the .bsxm file (as checklists or inside the taxonomy?)
• What to put in the custom checklists so that they export and import cleanly as .csv files?
• How to handle taxonomy upgrade?
• Should they come along with .btxm files? Seems at least somewhat elegant, since it lets custom taxonomies approach the state of built-in taxonomies (coming with a bunch of checklists).

[Adam Winer]

This happened some time ago. Not so much custom checklists, but built-in-to-the-custom-taxonomy. Which seems entirely sufficient.