Platform specific config

[joshuataylor]

With Postgres, we have Platform specific config.

E.g: unlogged should be a valid option in dbt_project, but setting this is an invalid option.

Supporting "official" adapters such as Postgres, Snowflake, etc would probably be the approach?

[joellabes]

@joshuataylor agreed - if you open a PR i'll happily approve it!