Add text to graph

Users should be able to not only annotate edges (#52 ) but also add a text block anywhere in the graph.

This could be used to:
- give a general description of the model
- give some details about a block

Potential tools for text editor:
- https://github.com/megahertz/react-simple-wysiwyg
- https://github.com/react-page/react-page/
- https://github.com/ianstormtaylor/slate 