ENH: add read_csv sniffing (sep=None) for C engine
If you now do `pd.read_csv(myfile.csv, sep=None)`, you get the warning:

> ParserWarning: Falling back to the 'python' engine because the 'c' engine does not support sep=None with delim_whitespace=False; you can avoid this warning by specifying engine='python'

Is this warning correct that it is not implemented for the c engine? 
If so, it could maybe be added.
