Add `get` method to get custom fields from node data

I'm proposing to add a getter for custom fields in node data. This allows e.g. store color of a rectangle, which can be then respected in `flowchart_template.html`.

Example of my usage:
- I store a type of a node and then render it in different color
![image](https://cloud.githubusercontent.com/assets/5679032/3687058/98c85494-1321-11e4-92c5-18d8519dfb8a.png)