changed react-fn to a multimethod

[zcaudate]

in reference to #46 suggestion that react-fn should be a multimethod.

[r0man]

@zcaudate Thanks for the PR! However, I'm not sure to open up this extension point in the public API (yet). I would like to wait until React 16 is released, and look into rendering JS literals before going down that route.
Also, what's the exact use case here? Usually I put smaller components into their own functions or a defelem.

[zcaudate]

@r0man: no worries. I was looking around at the compiler and was wondering how to do custom elements. I'm influenced by angularjs, where you can define your own tags and directives. I know that you can do defelem, but it would be really cool if they can be defined as a html tag.

[:custom-elem {} "hello"]