Introduce `\WP_CLI\Utils\interval()` and `\WP_CLI\Utils\time_since()` functions for working with human-readable time intervals
First pass at initial `wp cron` commands.

Available commands:

 * `wp cron event list`
 * `wp cron event run <hook>`
 * `wp cron event delete <hook>`
 * `wp cron schedule list`
 * `wp cron test`
Docblock updates
Switch from `\WP_CLI\Utils\format_items()` to the newer `\WP_CLI\Formatter->display_items()` method.
Output an empty table instead of a message when there are no cron events on the site
Remove some test code
Correct indentation
Rename `_list()` methods to `list_()`
Make `$time_format` a property on the `Cron_Event_Command` class
Introduce `wp cron event schedule` for scheduling new events
Remove `next_run` from the default list of event fields for now
Add the `--format` argument to our `list` commands
Update @nb's new export API with @dllh's fix for orphaned terms
Writer should use passed argument, not unknown variable
Docblock updates.
Examples for the `list` commands
Move `interval()` into `Cron_Event_Command` as a private method. Remove `time_since()` altogether.
Convert `next_run` and `recurrence` into optional positional arguments
Clarify that cron event times are in GMT
First pass at a Behat test for `wp cron`