(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.
Cool, thanks.  I think I'd just be a bit more explicit about "return a redirect response" rather than just saying "redirect it".