Create an API

Currently this program is designed to be used as an executable. It could also be accessible as an API, like how gollum-lib works.

* [ ] Figure out if it makes sense to have an API
* [ ] Design a Rust API
* [ ] Design an FFI-api
* [ ] Use the FFI-api to create a Ruby API that is a drop-in replacement for the Ruby `gollum-lib`.