Deal with sep= metadata line

[ngirard]

I recently used a tool which generates CSV files which begin with

sep=,

As far as I understand, this is Excel-specific and not part of RFC 4180.
It would be nice if Scrubcsv would reject this line, instead of silently accepting it if I interpret correctly the following test:

echo -e "sep=,\nA,B"|scrubcsv
(...)
2 rows (0 bad) in 0.00 seconds, 90.32 KiB/sec