Flag: playground url

[steveklabnik]

This issue is part of #125

--markdown-playground-url: control the location of the playground

Using this flag looks like this:

$ rustdoc README.md --markdown-playground-url https://play.rust-lang.org/

When rendering a Markdown file, this flag gives the base URL of the Rust
Playround, to use for generating Run buttons.

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Should we keep this flag or not?

[steveklabnik]

Given that there was only ever one alternative playpen, and now it's the official playpen, I'm not 100% sure if we should bother supporting this or not, instead always assuming play.rust-lang.org.

[steveklabnik]

We talked about this at the tools team meeting today.

It's not clear what to do here. There's only one playpen, so setting the URL is kinda weird, but at the same time, most crates won't work, so we don't necessarily want to turn it on by default.

I want to talk to the infra team to see what the plan is with regards to crates on the playpen before deciding how to move forward.