Wrap with quotes for ZSH support
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`
Throw an error if plugin(s) / theme(s) aren't specified for update

`wp plugin update` and `wp theme update` both require either passed plugin(s) / theme(s), or an `--all` argument. Our synopsis parser isn't smart enough to require one argument or another, so this will be the catch-all instead.
Merge pull request #1144 from wp-cli/fix-update-error

`wp plugin update` runs with missing argument,
If the sidebar doesn't exist yet, `array_slice()` will fail on a null value
Merge pull request #1140 from wp-cli/fix-1140

Widget Add - Warning: array_slice() expects parameter 1 to be array
Suppress errors when using a Widget's `update()` callback

Each widget can expect arbitrary array keys, and it's not possible to know what those array keys might be without processing `form()`. This is much easier for our purposes.
Merge pull request #1145 from wp-cli/suppress-update-errors

Error notices when using `wp widget add`
Make regenerating rewrite rules more user-friendly

* Throw a warning if the user is trying to regenerate `.htaccess` without the special declaration.
* Add directions on how to resolve the problem in the usage docs for each relevant command.
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
When scaffolding a theme or plugin, create the `themes` or `plugins` directory if it doesn't exist

For developers who use the scaffolding commands on brand new projects, this can save a moment of d'oh.
Important bugfix
Merge pull request #1151 from johnbillion/capital_p_dangit

capital_p_dangit
Rename `_list()` methods to `list_()`
Merge pull request #1149 from wp-cli/create-themes-dir

When scaffolding a theme or plugin, create the `themes` or `plugins` directory