More custom field revisions

[Myk Melez]

Gervase Markham wrote:

> The key thing is that this doesn't remove any abilities from the 
> proposal - those who want only single-product and global fields can 
> still have them. It also, I strongly suspect, makes the implementation 
> simpler because 1-product and n-product fields have more in common 
> internally.

Also, there's precedent for doing this and even sample code in the flag 
feature, so you don't have to start from scratch.  In fact, you might 
consider piggybacking on the current implementation for flags and 
factoring out its code for reuse in both flags and custom fields.

-myk