Add ability to commicate with nimble and `nim-toolchain.toml` toolchain override file to automatically select toolchain per-project

[evan0greenup]

For Rust langauge, cargo is capable to select toolchain automatically via rust-toolchain.toml file.

If the required toolchain is not installed locally, rustup is capable to automatically download it.

It would be nice to add the similar functionality in nimble/choosenim. So it can make nim software development (especially open source contribution) more easily.

With this feature, developer will not have to spend massive time on setup a compatible environment for Nim project.

@dom96 , @genotrance, what are your opinions?