Add WP_CLI commands for listing and updating flag states

It would be nice to have a cli command for quickly setting flags, or for viewing the state of all flags for a user or site.

This is easy enough to do already by accessing the user meta or options directly, but it would still be beneficial to add a little syntactic sugar around it. An example use case is when making a deployment to a multisite network which introduces new flags which have to be activated differently on the different sites.

The basic API structure I'm imagining is something like:

```
# List the active state of all flags on the current site
wp flag site list

# List the active state of all flags for a given user on the current site
wp flag user list

# Show the state of, and maybe details about, a specific flag
wp flag site get
wp flag user get

# Set the state of a flag
wp flag site set
wp flag user set

# Activate/deactivate all flags for a user or site
wp flag site set-all
wp flag user set-all
```