readData.geojson, readData.topojson

[aappling-usgs]

These file types can be read in as json, but the resulting format isn't ideal. These proposed readData functions could read data in as sp or sf objects. Of course, we'll need to decide which of those formats we want, and whether it's worth it for vizlab to make that decision on behalf of all vizzies.

I have mixed feelings about readData generally, which the above decision point raises again: are we really saving ourselves time or complexity when we force/strongly encourage the user to adopt vizlab's decisions about how and when to read data? It's not a baaad approach, just saying there are pros and cons.