Deprecate dataframe creation

[miriamasagold]

• The point of this package is to provide a lightweight convenience wrapper around the Congress API, so that users can get the data without needing to how how to make API requests.
• Because of this design goal, I use as few depends as possible (just utils, httr, and jsonlite). This helps avoid any back compatibility issues with the everchanging Tidyverse ecosystem.
• And yet, version 0.1.0 tries to provide a tidy project, with a return.data argument meant to create tidy dataframes for users out of the box.
• Having set this project aside for a year to see how the Congress API evolves, it's obvious that the interface is not stable. Almost all of the endpoints' contents have changed in ways that break the congress_data() function.
• This package's value is its lightweightness. Users that want a tidy object (with lots of tidyverse dependencies) can use Chris Kenny's package.
• In sum, congress_data() and its methods should be deprecated in favor of just returning either a) the raw response object, or b) the raw "content" dataframe within resp