Add init subcommand

[mattsse]

This adds a new subcommand init to create a virtual workspace.

a virtual workspace is a workspace where the root Cargo.toml manifest does not define a package, and only lists the workspace members.

This can be useful to quickly setup a Cargo.toml at the root of a mono-repo that includes nested cargo packages somewhere in its tree.

Example:

For:

app
└── rust
    ├── cli
    │   └── app
    │       ├── Cargo.toml
    │       └── src
    │           └── main.rs
    └── core
        ├── backend
        │   ├── Cargo.toml
        │   └── src
        │       └── main.rs
        └── command
            ├── Cargo.toml
            └── src
                └── main.rs

cargo ws init within the app directory will create the following Cargo.toml at app/Cargo.toml:

[workspace]
members = [
    "rust/cli/app",
    "rust/core/backend",
    "rust/core/command",
]

[pksunkara]

Please add the init command to the README and CHANGELOG. Also, since this command doesn't use git, you should be able to test this command. So, please add some tests too.

[pksunkara]

Merge these 2 lines please.

[pksunkara]

@mattsse Are you planning to finish this?