Support completion of subcommands and their arguments

[efung]

After installing this completion script, I realized that it doesn't complete subcommands, e.g. I expect dbt d<TAB> to offer debug deps docs, or, dbt run --s<TAB> to offer --select --selector --state as completions.

Digging around, I learned that the dbt CLI is built using Click and that it supports shell completion.

It would be nice to merge that generated script with this one to get the best of both worlds!