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.
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
Merge pull request #1149 from wp-cli/create-themes-dir

When scaffolding a theme or plugin, create the `themes` or `plugins` directory
Merge pull request #1147 from wp-cli/flush-notice

Make regenerating rewrite rules more user-friendly
Fuller PHPdoc coverage for `WP_CLI\Dispatcher\Subcommand`
Document `WP_CLI\Dispatcher\get_path()`
PHPdoc and explicit method definitions for `WP_CLI\Dispatcher\RootCommand`
PHPdoc coverage for `\WP_CLI\Dispatcher\CompositeCommand`
PHPdoc for `\WP_CLI\Dispatcher\CommandFactory`
More PHPdoc for `\WP_CLI\Runner`
`WP_CLI` and `WP_CLI\Configurator` also got some PHPdoc coverage
Merge pull request #1152 from wp-cli/internal-docs

PHPdoc coverage for `\WP_CLI\Dispatcher` and friends