WP CLI support

[jonny-bull]

Adds some basic CLI support for

• listing all flags
• activating one or a comma separated list of flags
• deactivating one or a comma separated list of flags
• toggling one or a comma separated list of flags

[jonny-bull]

Worth noting that there is a lot of room for expansion here.

The activate/deactivate/toggle commands should accept label and group arguments, allowing users to switch the status of a whole suite of flags at once. It should also be possible to change flag statuses across a WP network.

There should also be export and import functionality, allowing users to match the status of their flags across environments.