Support for CSV formatted results of subscriptions

[sgillies]

Resolves #933. The big choice here is to make CSV format access its own method instead of a flag on the existing method. I feel like this is warranted by the API split between JSON (paged) and CSV (not paged).

Sanity check @kevinlacaille @cpaulik ?

[cpaulik]

Thanks. Using this via planet subscriptions results UUID --csv makes sense to me.

Are we clear in the documentation about how to get the output into a CSV file? I think we have users for which that might not be obvious.

[sgillies]

@guyon-planet in the future, let me merge these. I have a couple changes to make yet.

[guyon-planet]

@sgillies, sorry about that