Add ability to set limit how far back FGift reads in PREVIOUS_YEARS_FILE

[sts10]

In some cases, users may have so long of a PREVIOUS_YEARS_FILE that FGift will not be able to make correct assignments, since too many matches will be prohibited.

To counter this, users could set a --lines-back flag to set how many lines (from the top) that FGift reads of PREVIOUS_YEARS_FILE. Crude, but it would be better than nothing (or maintaining a short PREVIOUS_YEARS_FILE and a separate file with the full history, for archival purposes).