Support toml rust-toolchain

[mkroening]

Closes #126.

This is a minimal version of #166 without the working directory change and the formatting changes.

[sdrll]

I think it could be merged to be consistant with rust documentation : https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file

[mkroening]

Closing since the project is dormant.