Support authoring rST directly in the input box

[waldyrious]

Currently we expect the user to paste or write rST formatted text directly into the plaintext textarea. We could improve on this experience by allowing some convenience rST authoring functionality to the textarea.

One example would be to implement syntax highlighting in the input area, which would signal to users when they are using valid rST syntax. I quite like the approach of LDT (Lightweight Decorator for Textareas), for instance.

Another way to support this flow would be to have an actual toolbar to insert formatting markup, as was the case in the now defunct http://rst.ninjs.org and its predecessor https://livesphinx.herokuapp.com. I'm not so sure about this one, though, as it could drastically increase the complexity of this tool.