Dbt-impala

Added
- EXTERNAL table support 13
Fixed
- Schema change on VIEW models incorrectly runs ALTER TABLE and fails 20
- Column type overrides not honoured on dbt seed 18
- Remove unsupported backup clause 15
Known issues
- Possible inconsistent failures on subsequent runs for views 23

Adds:
- Kerberos Auth
- dbt docs
- dbt snapshots
- dbt 1.0.4 support

Fixes:
- dbt --version not working

Supports:
- LDAP Auth
- Table & View materialisation
- Incremental append & insert_overwrite
- dbt seeds
- dbt tests