Allow translators to insert measures without hardcoding them into generic code

[toobaz]

Currently, any metric added by a translator needs to be inserted (possibly deactivated by default) in _auto_stats in stargazer.py.

Ideally, this wouldn't be needed, as Stargazer would automatically pick up any metric defined for a given model.

This is not difficult but requires translators (and possibly) users to be able to indicate the position of the metric in the list at the bottom of the table, as otherwise it will be a mess.