[Feature request] Allow headers to be defined in fenced block

[pdfkungfoo]

• Allow the table header line to be defined inside the fenced code block. For example by supporting an additional options for the header attribute: header="fenced" and header="line 13".

This is useful (also in combination with my other feature requests) when you have rather large CSV files, but you do not want to change the headers (permanently or temporarily) just for the table inclusion in a Markdown/Pandoc document. (Some headers are fine for processing/maintaining the CSV table in a spreadsheet, but may not be good in their original version in a published document.)