Add a "dev mode" variable to toggle on materialization of stg and int models

[katy-sadowski]

Currently, all stg and int models are materialized in the database. This clutters the schema and is not desirable for production analytic use cases. However, developers (and analysts in some cases) may want/need to inspect the contents of these models.

We can use a dbt_project variable that users pass into the command line that will enable materialization of stg and int models (I think it should be disabled by default).