Add `wp plugins-api` command

WP-CLI command for getting plugin information form WordPress.org Plugin API.

* `wp plugins-api author <author>` - Get a list of plugins for specific author.
* `wp plugins-api browse <browse>` - Get a list of plugins for popular/new/updated/top-rated.
* `wp plugins-api info <slug>` - Get a plugin information specific plugin.
Merge pull request #35 from miya0001/patch-1

Add `wp plugins-api` command