Larger margins / lower text width / style

[jstac]

The long text lines make reading tiring. See, for example,

http://nbviewer.jupyter.org/github/QuantEcon/QuantEcon.notebooks/blob/master/IntroToStan_basics_workflow.ipynb

Can we control this in the forthcoming version of the site? I guess so: People submit ipynb files and we add some CSS that gives them a common format / style?

If this is possible it would be good to do, and the first thing we should do is have larger margins, so the lines are shorter.