Add `--generate-config-from=file` and support common package formats

[chasinglogic]

Many programming language package managers support a meta data format. Some examples include Cargo.toml, package.json, and setup.py. They all use SPDX identifiers and have author information.

To help with adoption of Licensure it would be nice if we could make a new config generation flag that takes these package metadata files as input and generates a licensure config from the information present there.