Add a way to install the toolchain specified in the override file

[shinglyu]

#1172 allows us to create a rust-toolchain file and specify a version of the toolchain to be used in the current directory. But I can't seem to find a way to install the specified version automatically.
Now I do cat rust-toolchain | xargs rustup install, but it would be nice to have a command like rustup install rust-toolchain or maybe simply rustup install (fail if the file is not present).

Thoughts? @brson

[paulrouget]

As of now, what happens exactly when the toolchain version is not available? Just fail or fallback to the closest toolchain?

[paulrouget]

Just fail.

[shinglyu]

Oh sorry for missing this. Yes it just failed with a warning saying the version didn't match