Cron commands

Turns out I'd done a bit more on this command than I thought. Here's the first pass.

## Available commands

 * `wp cron event list`
 * `wp cron event run <hook>`
 * `wp cron event delete <hook>`
 * `wp cron schedule list`
 * `wp cron test`

## Notable omissions

 * `wp cron event add ...`
 * `wp cron schedule add ...`
 * `wp cron schedule delete ...`

Feedback welcome, especially with regard to which fields we should return in the `wp cron event list` command (currently it returns three fields relating to the date).