set default style with min-height & word-bread

[gfortaine]

@mac-s-g Many thanks for this great project. We are successfully using it in production. However, we have identified 2 minor bugs :

• the AddKeyRequest component is masked when the initial JSON is empty and there is another ReactJson component below. So, a good compromise could be to set a min-height on the react-json-view container
• when the value of a key is too long, after a click on it, it displays beyond the container. So, a good fix would be to use the word-break CSS property

[gfortaine]

#240

[mac-s-g]

thank you!