Some CSV files are given with an header line containing the list of
their column names, use that when given the option "csv header".
Note that when both "skip header" and "csv header" options are used,
pgloader first skip as many required lines and then uses the next one as
the csv header.
Because of temporary failure to install the `ronn` documentation tool,
this patch only commits the changes to the source docs and omits to
update the man page (pgloader.1). A following patch is intended to be
pushed that fixed that.
See #236 which is using shell tricks to retrieve the field list from the
CSV file itself and motivated this patch to finally get written.