Add max distance tracking capabilities

Added 'maxDistanceTracking' and 'maxDistanceFormat' as options to 'amTimeAgoConfig'.  Set maxDistanceTracking to a number of days at which you do not want to show "x days ago" format, but would rather show the format specified in "maxDistanceFormat", which uses Moment.js formatting.  Also, do no watch values which are not in "time ago" format.