(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.
btw this is a change in behavior compared to what we were doing before, right?  So this'll necessitate a "major" version bump rather than a "minor" one?  No changes in the code related to that, just asking so that we'll know what to do when we're ready to release it.