Move env.cljs; Add `spliced`; Use `spliced` in `reactive-attributes`

- The `spliced` form splices its children into its parent:
  Ex. (div (spliced (h1 "hello") (p "test")))
      => (div (h1 "hello") (p "test"))
- This is especially useful for macros. See `reactive-attributes`.
Merge branch 'master' of github.com:tailrecursion/lein-hoplon