Allow specification of different placeholders for different kinds of mask.

Allows this:
$.mask.definitions['d'] = $.mask.definitions['9'];
$.mask.definitions['m'] = $.mask.definitions['9'];
$.mask.definitions['y'] = $.mask.definitions['9'];
$.mask.placeholders['d'] = 'd';
$.mask.placeholders['m'] = 'm';
$.mask.placeholders['y'] = 'y';
$('.date-field').mask('mm/dd/yyyy');

also adds a class "placeholder" when the element is empty