Feature: Skip Activity Stream

[bcodell]

Resolves #31 Resolves #36

This PR:

• adds support to configure skipping activity streams in a given activity schema pipeline, including:
  • new optional config parameter in dbt_project.yml
  • expands support for cluster_keys macro to be used in streams and activity models
  • updates the build_activity macro to build an empty table when skip_stream is true for the corresponding activity schema
  • updates the dataset macro to select from activity models rather than the strem when skip_stream is true for the corresponding activity schema