Added a new HBSPlug which is kind of a generalisation of HBPlug (should
still use HBPlug for the Humanity Library collections). Also removed the
old GB specific GBBPlug, GBHPlug, and GBTPlug plugins. Can now do what
these plugins once did by giving the "-input_encoding gb" option to
HBSPlug, HTMLPlug, and TEXTPlug respectively (although TEXTPlug doesn't
actually take this option yet -- I'll fix it soon).