Add WP-CLI commands

[dlh01]

Description

Add WP-CLI commands for accessing the data in The Big Pit, probably including the core methods:

• get()
• set()
• delete()
• flush_group()

As well as a list_group() or similar command for a paginated listing of the items available in a group.

Use Case

Developers should be able to inspect the contents of the table like they can posts, terms, etc.