rustdoc: add cli argument `--playground-url`
Add a new cli argument `--playground-url` for rustdoc:
`rustdoc lib.rs --playground-url="https://play.rust-lang.org/"`
`rustdoc book.md --playground-url="https://play.rust-lang.org/"`
This makes it possible for tools like https://docs.rs to generate crate docs that can submit samples code to run at https://play.rust-lang.org, even if the crate's author *forgot* putting `#![doc(html_playground_url = "https://play.rust-lang.org/")]` to crate root. By the way, I'd like to say, many crate authors are not aware of existing of `#![doc(html_playground_url = "https://play.rust-lang.org/")]`.
`--playground-url` may be reset by `--markdown-playground-url` or `#![doc(html_playground_url=...)]`, so it's backward compatible.
@alexcrichton since you implemented playground-url related features.