Refactor config package and add functionality

[samcoe]

This PR introduces a bunch of functionality necessary for gh to utilize go-gh for config handling.

Things changed:

• Renamed and improved configmap to yamlmap
• Exported config package with functions for manipulating configuration files. Only business logic that is encapsulated is around reading and writing of configuration files.
• Exported auth package with functions related to auth tokens and hosts. This functionality is not new, it just used to live in the internal config package.

I added inline comments and questions for design decisions.

Note: This PR is not as long as it seems, lots of code was just shuffled around.

cc https://github.com/cli/cli/issues/5560

[samcoe]

@mislav This is ready for re-review when you get a chance 🙇