Add a "start" button + parameter

[jdevalk]

To allow embedding the playground in places where you don't want it to autoload the entire playground immediately, I'd like to add a start_button parameter, defaulting to 0 or off. If set to 1, it should give a simple start button that needs to be pressed before loading the entire WordPress playground.

This has already been coded into the playground embedder plugin.

[adamziel]

Shipped in #537 via ?lazy=1