Recurse when handling nested lists

This renders correctly:
```clj
[:div {} [:br] [:br] (list [:br] [:br]]
```
but this does not:
```clj
[:div {} [:br] [:br] (list [:br] (list [:br])]
```
Isaac and I discussed. It seems reasonable that these can be flattened so that everything becomes a child of the parent element.

https://github.com/dmohs/react-cljs/blob/master/src/main/cljs/dmohs/react/core.cljs#L108