Cells support

[jurrick]

Fixes #315

[nathanvda]

Thanks. This is not a huge priority for me. As I have no experience with cells, also a bit hard to confirm this works. Also no documentation is added, so without documentation nobody will know how to use it.

I have to do some catch-up with all the open issues/PR's first 😄

Not sure if this is the best approach, an alternative approach would be to split this up in two gems, where I include your changes to ViewHelpers so it can be overridden, and a cocoon-cells gem (or cells-cocoon, whichever makes more sense) where the CellsHelper lives, with specific documentation.

I hope to find some time this weekend to work on cocoon.