feat: allow passing non-default features

[balins]

This PR allows for passing additional features for the package (either space or comma-separated, multiple occurrences of --features [...] are allowed). This allows us to pull licenses for optional dependencies. Notice that the default features are always enabled.

If at least one non-existing feature is provided, the CLI will exit with an error message:

cargo bundle-licenses --features somefeature,nonexistent --output THIRDPARTY.toml
Error: `cargo metadata` exited with an error: error: the package 'package' does not contain this feature: nonexistent

Similarly, passing empty feature list will error out as well:

cargo bundle-licenses --features --output THIRDPARTY.toml
error: The argument '--features <features>...' requires a value but none was supplied

USAGE:
    cargo bundle-licenses --features <features>... --format <format> --output <output>

For more information try --help

(I guess this is desirable behavior to catch interpolating empty values in scripts etc.)

While this isn't a breaking change for the CLI itself (default features are preserved and the new option is, well, optional), this is a breaking change for the cargo-bundle-licenses as a library. If this is an issue, we can consider preserving the old API and creating separate methods / structs that the old API will proxy to.

Fixes #47

[sstadick]

@balins this is great! I'll get it merged in and released tomorrow morning.

RE backward compat for using this as a library, I don't think anyone uses this that way, but to be on the safe side we'll make this a major version bump.

[sstadick]

This should be in the 3.0.0 release