implement steve-cmd ls

[willkg]

Probably want to use blessings to get colors and tables and such.

Might even want to specify which fields you want to see. So, it defaults to:

• title
• whiteboard

but allows you to specify the fields explicitly.

[willkg]

Don't use blessings. Do use prettytable (https://pypi.python.org/pypi/PrettyTable).

[willkg]

We're using tabulate already, so we don't need to add PrettyTable.