Integrate wp-cli directly on playground.wordpress.net

[adamziel]

With the loopback handler we can just add a button to show the terminal (quake-style console from the top of the screen, add keyboard shortcut). The first run downloads wp-cli if missing with a progress bar. Then you get to use wp commands.

[adamziel]

Actually, better yet - let's make it a WordPress plugin and enable using it in any Playground. There's no need to restrict it to one specific web app.