Add meta to cells

[mynomoto]

• Add a new arity to cell so we can add metadata when creating the cell (same as atoms)
• Implement cljs.core/IWithMeta to be able to create cells with meta
• Tests

[mynomoto]

This improves over https://github.com/tailrecursion/javelin/pull/21

[alandipert]

:metal: :panda_face: :metal: Thanks!