expand FilterMatcher functionality with GroupBy settings

[geo-martino]

Begins to resolve #18 by clearing up some overlapping logical errors in ItemSorter and FilterMatcher

Added

• Extend :py:class:.FilterMatcher with group_by tag functionality

Changed

• :py:class:.ItemSorter now shuffles randomly on unsupported types
  • prioritises fields settings over shuffle settings

Removed

• Redundant ShuffleBy enum and related arguments from :py:class:.ItemSorter

Documentation

• Added info on lint checking for the contributing page