postgresql database creation

Looks like a postgresql database must first be created before new_database can be run:

```
./taxit -v new_database postgresql://localhost/ncbi_taxonomy
...
sqlalchemy.exc.OperationalError: (psycopg2.OperationalError) FATAL:  database "ncbi_taxonomy" does not exist
```

Is there a reason not to auto-create, or at least provide a flag to do so?