Add possibility to run workspace wide scripts

[manu0466]

This PR implements the possibility to define scripts inside a Cargo.toml file of a workspace. The syntax is similar as the one of a package. Here an example of Cargo.toml of a workspace that defines a script called hello

[workspace]
members = [
    "memmber1",
    "member2",
    ....
]

# Scripts
[workspace.metadata.scripts]
hello = "echo Hello"

[JoshMcguigan]

Thanks for including the example of this in the PR description. Could you also update the README?

[maurolacy]

@JoshMcguigan, can you merge this and create a new release? Thanks!

[maurolacy]

@JoshMcguigan are you there?

[JoshMcguigan]

Thanks @manu0466 for the contribution and @maurolacy for the review and ping!

I've published this as version 0.2.0.