update read_json to be consistent with readr

[jeremystan]

From hadley:

I think resolving that is actually really simple — just define read_json(), which is more tidyverse like anyway. Ideally this would either take regular json and return a 1 row tbl_json, or ndjson (and other common multi-json formats) and return multi row data frames.

tidyjson has a read_json already, but it is old. I will revisit readr and perhaps integrate this more closely into the package documentation.