Label Support

[rhdunn]

It would be useful to label cards, especially when tracking a lot of different types of task on the card (e.g. feature request, bug, ui/ux, and performance).

1. Add/remove labels on cards.
2. Add/remove labels on lists (a different set to the ones on cards).
3. Select the background/foreground colour for the labels.
4. Create new labels in-place.
5. Auto-complete for existing labels in the list select drop-down.
6. Group by label for lists with a configurable ordering.
7. Label hierarchy -- e.g. release/[VERSION], feature-request/[PRIORITY].
8. Archived lists/cards could be implemented by having labels have an "archived" flag and a default "Archived" label with that flag set.
9. Filter the view to show selected card/list labels. (This could be how the archive support is implemented, i.e. a default Archive flag is hidden by default.)

The idea behind the list grouping labels is to be able to organize cards in a way that keeps the information when moved into completed lists, and to better organize long lists.