Allow two et al. numbers, for print and for threshold

[opoudjis]

At the moment, we have a single number threshold for displaying "et al.": If there are more than N authors, display the first N authors, followed by "et al."

In fact, we need two numbers: a threshold triggering "et al.", and the number of authors to display. That is how Chicago Manual of Style (followed by IEEE) works:

For more than ten authors (not shown here), list the first seven in the reference list, followed by et al.

If the print number is not supplied, it is the same as the threshold number.

[opoudjis]

So we will have:

• etal_count: as now, the number of authors to trigger et al. in bibliography
• etal_display: how many authors to show if using et al. in bibliography (by default, same as etal_count)

[opoudjis]

@reeseplews FYI, the correct behaviour for IEEE et al. will be available next Metanorma release (in a week).