(TK-83) Add :enable-trailing-slash-redirect option

Add new :enable-trailing-slash-redirect option to the options map
for all service functions that add handlers. When true, all
requests made to the handler without a trailing slash will be
redirected to have a trailing slash. Defaults to false.
@fpringvaldsen agreed that the name I suggested was misleading.  How about `redirect-and-add-trailing-slash`?