[csv-] warn when guessed option differs from default

[midichef]

When csv is guessed without a filetype, visidata uses Python's csv library to guess the parsing options using csv.Sniffer. For the simplest csv files, it seems that doublequote is False, whereas the default in visidata is True. That can surprise users, as it did in #2689.

This PR makes the guessing process more obvious. It outputs a warning for all guessed CSV options that differ from visidata's defaults:

echo a,b |vd
...
guessed option csv_doublequote=False
guessed option csv_skipinitialspace=False

(versions that do the silent guessing are 3.0 and up)

Or should we override the Sniffer guesses to use visidata's defaults?