Better integration with `cargo-deny`?

**Is your feature request related to a problem? Please describe.**
Right now, `cargo-deny` and `cargo-about` are two separate tools. This is fine, except that both tools will validate the license files used in the project.

As such, the list of accepted licenses needs to appear in both `about.toml`'s `accepted = []` _and_ `deny.toml`'s `allow = []` arrays.

**Describe the solution you'd like**
The simplest solution I think would be for `cargo-about` to not need an `accepted` array and simply spit out _all_ found licenses. Or at least having a flag to do so. This way, I could use `cargo-about` to generate the license file and `cargo-deny` to enforce license requirements.

**Describe alternatives you've considered**
Maybe the two configurations could be merged? For example `cargo-about` could read the `deny.toml` file and pick up the allowed licenses in there. `about.toml` contains more config options, so those would need to be moved to `deny.toml`. That's a large breaking change though.