(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.
(TK-83) Address PR feedback

Change name of `:enable-trailing-slash-redirect` option to
`:redirect-and-add-trailing-slash`. Add more explicit
explanation of this option to README.md.
(TK-83) Address further PR comments

Change name of setting to `redirect-if-no-trailing-slash`.
Change `add-context-handler` to take an options map instead of
the `follow-links?` and `enable-trailing-slash-redirect?`
arguments. Don't negate `:redirect-and-add-trailing-slash` option
when it is being passed around the code.
Merge pull request #87 from fpringvaldsen/TK-83-trailing-slash

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