Programmatically add a catalog to Intake

On https://intake.readthedocs.io/en/latest/quickstart.html#adding-data-source-packages-using-the-intake-path I read `Adding Data Source Packages using the Intake path: Intake checks the Intake config file for catalog_path or the environment variable "INTAKE_PATH" for a colon separated list of paths (semicolon on windows) to search for catalog files. When you import intake we will see all entries from all of the catalogues referenced as part of a global catalog called intake.cat``

1. Should the title be `Adding Catalog Packages...` instead of `Adding Data Source Packages`.
2. Is there an API to add a catalog to intake? (*)

(*) eg.

```py
cat = intake.open_catalog('us_states.yml')
intake.add_catalog(cat) # ???
```