Rustup integration

[da-x]

Today rustup brings the standard documentation with the toolchain, as HTMLs. There is even a rustup doc command to open the browser over the current toolchain documentation. There was an unmerged PR for adding a --path to rust doc so it would just print the path where the HTMLs are installed.

If rustup gets improved and rust-doc.vim would somewhat rely on it, then we would not need the instructions in the README.md for manually downloading and installing the standard library docs, which will reduce burden from users.

p.s. rust-doc.vim looks awesome, I think it really improves working with Rust under Vim. I am currently maintaining rust.vim, and I consider to attach a list of recommended additional plugins in the main README.md. This would be definitely one of them :)

[da-x]

Pending rustup change: rust-lang/rustup#1453

[da-x]

The PR adding --path was merged into rustup.