Support multiple color labels

[philipmjohnson]

Currently, labels come in one color. Multiple users have requested support for multiple label colors. However, label colors must be custom to each Morea theme. So, let's propose that each theme should provide three colors for labels, called label-color-1, label-color-2, and label-color-3. The YAML front-matter should be augmented to support a new syntax for labels:

morea_labels:
  - ["this is the label", label-color-1]

This syntax allows the user to explicitly specify the label color. Note that when you want label-color-1, you can use the traditional syntax instead. So the following is a shorthand for the above:

 morea_labels:
   - "this is the label"

You can mix and match:

morea_labels:
  - "my first label"
  - ["a different color label", label-color-2]
  - "another label"