Compatibility with CSV files that have no headers

These tools work without changes:

* csvclean
* csvjoin

The following had specific edits:

* csvstack (the reason for this issue!): don't eat the first row of the 2nd, 3rd, etc. files
* csvlook: don't render first row as a header

The following had some options that are incompatible:

* csvpy: I doubt anyone wants fake column names when using --dict, so we just raise an error
* csvcut: --names conflicts with --no-header-row
* csvgrep: --names conflicts with --no-header-row
* csvsort: --names conflicts with --no-header-row

Still left to do:

* csvsql
* csvjson
* csvstat