Unify Implementation of RenderLeaf and RenderElement

[lukesmurray]

The implementation of renderElementPlugins is pretty straightforward.

Each plugin defines a function which returns the element.
If the plugin does not manage the element then it returns undefined.

The implementation of renderLeafPlugins is confusing in comparison!

Plugins have no way of opting out of render leaf.
If a plugin does not manage a certain leaf then whatever it returns overrides the children prop.
Also the plugin should be able to return the entire leaf, not just the children!

Is there any reason for the difference in implementation?
Happy to provide a PR with a renderLeafPlugins implementation closer to the renderElementPlugins implementation.

[lukesmurray]

nvm can just return children! I understand this is to allow nested leaves.