Use enabled config to implement cross-database support

[drewbanin]

Right now, Snowplow models are encoded as macros in order to implement cross-database support. Instead, we should experiment with making models specifically for each support adapter. I think broadly, this falls into:

• default (snowflake/redshift/postgres)
• bigquery

The models directory might look like:

models:
 - common/
 - default/
 - bigquery/

Every model would need to have a config like:

{{ config(enabled=(var('adapter') == 'default')) }}
or
{{ config(enabled=(var('adapter') == 'bigquery')) }}

I don't think we can configure this at a folder level in dbt_project.yml, but it might be worth exploring if there's some clever way to implement that.

The results of this experimentation might result is us wanting to keep the adapter macro pattern, but I'm interested in exploring alternative ways of supporting cross-db model implementations in packages!

[jtcohen6]

Resolved in #62. Once 0.16.1 comes out, and dbt supports returning boolean YML values from Jinja macros, we can pursue an even-more seamless implementation in dbt_project.yml.

[drewbanin]

Just an FYI - boolean yaml values will likely be 0.17.0, but regardless this all sounds great!