Added column-completion option

Added "force_cells" option which causes extra cells to be created on each row
to match $max_rows. Useful if you're exporting from several spreadsheet apps
(including Google Spreadsheets) that don't create cells if you have empty
trailing columns.

So for example, if I have a CSV like this:

<csv>
name,phone,email,im
filipe,4085552345,filipe@mercenary3.com,filrprogger
cosmocode,5145551234,cosmocode@spam.la
</csv>

Because the 3rd line doesn't have a 4th column, the current CSV plugin will not render a 4th column for that row. On longer spreadsheets, this can look pretty ugly. By doing:

<csv force_cells=1>...</csv>

This will now force the CSV plugin to add extra columns to each row up to $max_rows in syntax_plugin_csv::render() so that you end up with a complete table. I can provide an example in action if this doesn't make sense.