Add wp cli commands to get stats and delete stats

[pattonwebz]

This adds a setup to handle WP-Cli commands and adds a pair of commands to start with. It scopes them under a accessibility-checker command scope.

• delete-stats request a post ID passed as the first positional argument
  • Delete stats for post with ID 123: wp accessibility-checker delete-stats 123
• get-stats accepts a post ID passed as first positional argument, passing no ID will return site stats.
  • Get stats for post with ID 123: wp accessibility-checker get-stats 123
  • Get stats for entire site: wp accessibility-checker get-stats

Closes: #674