Rename root `host` attribute to `vagrant`
Implement root#eval_vagrant method
Add loader prototype
Support .yaml and .yml file extensions
Overhaul model classes to handle safe lazy attr loading
Update synced_folder to use new attribute syntax
Update model/root handling of vagrant attribute
update model/network/private_network attr syntax
Convert model/network/forwarded_port to use new model syntax
Merge branch 'sloth_loading'
Rename ModelCollection to generic ClassRegistry
Correctly handle .yaml and .yml files in yaml loader
Add basic ConfigBuilder.load method

It's a trivial implementation to get an MVP out of the door.
Merge branch 'registry'