Datetime driver

 * Replaced `Cron` driver with `DateTime` driver
 * Reworked verbiage on driver documentation to better communicate their purpose
 * `DateTime` driver has these added methods: `quarterly()`, `weekly()`, `week()`, `weekOfYear()`, `everyOddWeek()`, `everyEvenWeek()`

This this pull request includes breaking changes, I'll leave this open for 24-48 hours for comments/feedback before merging it in.
Unfortunately we can't.

This is what we need: https://github.com/wp-cli/php-cli-tools/commit/02d5ed4cdbab911df91bf06062c8b2705fb0919a#diff-5a154f2370854dec85848f1f76b4bc3cL26

This is what it looks like in master: https://github.com/wp-cli/php-cli-tools/blob/master/lib/cli/Shell.php#L27