API additions to support Ohai

Ohai needed a few API additions to ADNKit, and they are included here.

- Support for filtering annotations by type on ANKAnnotatableResource
- Support for requesting machine-only posts on ANKClient
- Support for requesting files using specific read-only tokens
- Support for fetching subscribed ANKChannels, limited to specific types

I've tried to maintain the coding styles and patterns the project already uses, but I'm happy to make any changes desired.