#637 General modifier for key pattern / field marker :IniN

[Paul van Klaveren]

Based on http://jabref.sourceforge.net/help/LabelPatterns.php &
Feature request ID: 2589382 (https://sourceforge.net/tracker/?func=detail&aid=2589382&group_id=92314&atid=600309)
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Looking at all the existing fieldmarkers ending with "IniN", it would be a more
elegant solution to create a single modifier that takes the first N characters of
the text produced by the field name or spcial field marker.

Suggestion for modifier name is :IniN