Command help formatting

[nikhilk]

For an example, see https://github.com/GoogleCloudPlatform/datalab/blob/master/content/datalab/tutorials/BigQuery/BigQuery%20Commands.ipynb

Try

%bigquery --help

1. Output seems arbitrarily constrained to some number (80?) columns, causing word-breaks on character boundary.
2. Subcommands are listed as positional arguments which is odd.

Can we take over help for commands and provide a better experience? Seems like argparse isn't well suited to producing the kind of help we want, and if I recall there are ways to customize it without completely re-doing it ourselves.