Plugin set for project manifests

[spenserblack]

It would be cool if there was a set of plugins for reading manifests and reporting the project
type (package.json => NPM, Cargo.toml => Crates.io, etc.).

This won't be part of this repo, but a separate repository, making it a collection of 3rd-party
plugins.

Before these can be implemented, the main gem must support the following, because building from a git
repository is too biased (and also cumbersome to reverse back into a path to find manifest files):

• matches_path?
• from_path