Polish the Hierplane integration into the AllenNLP demo.

[codeviking]

There are some UX / UI issues associated with the integration of Hierplane into the AllenNLP demo. We should:

• Fix these.
• Add options / incorporate changes into the library itself where appropriate.

[codeviking]

This can't be accomplished until allenai/allennlp#526 lands.

[DeNeutoy]

Adding a few minor style things here for the polish:

Is it possible to make the text inside the boxes wrap, so that the full tree fits in the space?

The box containing the passage could be bigger, so that you can see all of the text for long sentences without having to scroll.

[aaronsarnat]

Seems like these problems have been addressed and this issue can be closed. @codeviking would you concur?

[codeviking]

Yup, we're good to go in this regard!