Spawn a playground instance per Rust PR merged

[lqd]

It could be useful to spawn an instance of the playground for a specific rust PR, especially for the more user-facing of changes.

Testing out and easily showing others, concrete changes to ergonomics, or similar — work about NLLs, chalk/traits, self method calls, macros, @mbrubeck mentioned testing the #[async] / generators branches as well — before they're integrated in nightly could be very helpful.

As anecdote, Babel.js seems to have such a feature, an instance of their REPL built per PR, and they seem very happy about it :)

[shepmaster]

Relevant:

Is there a public online playpen with this to play with it before it is merged to nightly?