Expose a library API

This is not usable as a library dependency for others via `go get`. Can keep the CLI iface the same, just pull the code into a package and expose things with reasonable names. Everything that can be done via the CLI should be doable programmatically (of course, the CLI code just invokes the lib code). I'd send a PR to do this myself, but I figure such a large refactor (as in size, not effort) should be done by the maintainer.

(also, during the refactoring, fix things like exposed internal variables, use of bool returns instead of error, etc)