This PR contains a struct to allow you to generate JSON schemas from
Golang types and a struct to allow injecting documentation into the json
schema. This will support autocomplete for DABs
This adds:
* Top level "artifacts" configuration key
* Support for notebooks (does language detection and upload)
* Merge of per-environment artifacts (or artifact overrides) into top level
Load a tree of configuration files anchored at `bundle.yml` into the
`config.Root` struct.
All mutations (from setting defaults to merging files) are observable
through the `mutator.Mutator` interface.