fread could attempt auto-unzip

[MichaelChirico]

Spurred by: https://stackoverflow.com/q/57829746/3576984

Wikipedia gives zip file signatures: https://en.wikipedia.org/wiki/List_of_file_signatures

And indeed the file in that SO Q has the zip signature:

readBin('household_power_consumption.zip', raw(), 4L)
# [1] 50 4b 03 04

Since unzip is standard (utils) functionality, it seems reasonable we could auto-unzip files for convenience.

Error if nrow(unzip(f, list=TRUE)) > 1L) for now, but maybe more advanced once #586 and/or #2582 are done.