Add an optional description field

[Fl0Cri]

It would be nice to be able to attach a description to a tag. It could be displayed on the page listing all posts attached to a tag.

Actually it would be easy to do a plugin that would add this feature (not tried yet), but I think it would be great to have it out of the box.

[scottbedard]

I would accept this PR. If you (or anyone else) decides to do this, please set it up as a description_html field which uses a richeditor, and a description_plain field that is auto-populated when the model is saved. This model from another plugin of mine demonstrates what I'm talking about.