Ability to cargo install <dependency>@<version> should update Cargo.toml

[shaunwarman]

Warning: New to rust.

Problem
It seems to add a dependency using cargo to my rust project, I need to manually alter Cargo.toml directly. For bootstrapping projects, this seems a bit slow and cumbersome.

Solution
It would be great to just cargo install <dependency>@<version> or cargo install <dep_1> <dep_2> <dep_3> where dependencies 1 thru 3 are installed as the latest and Cargo.toml is updated appropriately.

Instead of README's adding:

# Cargo.toml
[dependencies]
<dependency> = "<version>"

Make it easy for consumers of the crate via:

cargo install --save <dependency>@<version>

[carols10cents]

You can do this today with the cargo-edit crate! There are plans to integrate cargo-edit into Cargo proper, which is tracked in #5586, so I'm going to close this in favor of that issue. Thanks!