Ability to define foreign data wrapper source and schema

It is sometimes useful to connect to foreign data sources in a data warehouse. For example, pulling in data from a remote source. The Postgres foreign data wrapper allows such connections to be made. However, the foreign data wrapper would be invisible to `dbt` and so might not be included in standard tests and lineage graphs.

Please consider adding support for foreign data wrapper sources and schemas in order to include them in a comprehensive data warehouse design.

Original discussion: https://github.com/fishtown-analytics/dbt/issues/2182