Latest rustup added toml format for `rust-toolchain`

[kinnison]

Hi, rustup developer here,

The latest rustup release (1.23) added support for the rust-toolchain file to contain structured data - see https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file for information.

I think that if you're parsing rust-toolchain you'll need to add support for this.

If you do, then feel free to give me a poke if the semantics aren't clear. Also I intend to augment this with further capabilities in the future, so if this kind of issue is useful I'll ping further ones when we merge additional functionality.

Thanks,

Daniel.