Addition of an unzip argument to optionally not unzip downloads

[Aariq]

This is a draft addressing #124. Things I'm still uncertain about, but would be happy to add to this PR:

• How this should/shouldn't interact with process_zip() that deals with pre-1980 files
• Where to put a test for this (seems like tests in test-prism_webservice.R purposefully don't hit the API)
• Would it be helpful to document how to read in data from a .zip without unzipping? E.g. in an example or short vignette?