Find closest Cargo.toml to your current file instead of looking in the project root

[donaldpipowitch]

Hi there!

In celebration of Rust 2018 I update my "Rust for Node Developers" tutorial. This project contains several standalone example which have their own Cargo.toml (e.g. here). But without a Cargo.toml in the root of my project nothing works (not even formatting the source code). I can workaround this by adding a workspace Cargo.toml in the root, but it is really a workaround, because the packages don't depend on each other. And I even have a small example without any Cargo.toml (e.g. here). In this case I'd expect basic features like formatting to work.

So my which list:

1. Find the closed Cargo.toml relative to my current .rs file by recursively looking in parent directories.
2. If a Cargo.toml can't be found some default/fallback behaviour should kick in so things like formatting still work.

Thank you.