Add support for dbt version 0.21.0

[tomasfarias]

The release of dbt version 0.21.0 comes with a lot of goodies, in particular, the following ones should be supported by us:

• dbt source snapshot-freshness renamed to dbt source freshness.
• New artifact schemas.
• New dbt build command.
• --output-keys parameter for dbt ls.
• Switch to --select instead of --models in all commands. We should keep the models parameter for backwards compatibility though (maybe joining the two?).