Add option to filter licenses by top-level project features

[jwodder]

Currently, cargo-lichking determines the packages to bundle licenses for by evaluating the dependency tree for the top-level project with its default features selected. However, it may happen that a developer wants to distribute binaries built with a non-default feature selection (instead of or in addition to binaries with default features), and so they would need a license bundle that reflects the dependencies added & removed by the change in features, but cargo-lichking does not provide that.

I therefore request the ability to specify a set of features to apply when determining a project's dependencies.

[Nemo157]

@jwodder before you get too deep in with cargo-lichking, I should mention I've been meaning to explicitly deprecate it for a while. There are other existing tools that cover most of its functionality better.

[jwodder]

@Nemo157 I figured it was inactive. I'm not actually going to use cargo-lichking; I'm writing up a comparison of Rust license-bundling programs, which means judging the quality of each one, and I'd be remiss if I didn't file issues for the problems I found.