Taxonomy list and autocompletion

[chriskrycho]

As a user of lx, I want:

• to be able to get a list of the existing instances of each taxonomy, e.g. every existing category or tag name.
• to have autocompletion from the list of existing items in a taxonomy when I've typed the relevant command flag (e.g. if there is a --category flag, it should then allow auto-completion for existing categories)

Note that the latter is closely related to #33, and that it depends on the former.