Updated so that only one function needs to be overridden.

This will make using lfe-reveal-js from another repo much easier. For instance,
all one has to do now is:

1) create a new project
2) add lfe-reveal-js as a dep
3) create a module with an 'out' function defined in it
4) have that function call lfe-reveal-js-routes:routes, passing a function to
   call which will generate the content
5) write a content-generating function.