HTML build step

[choldgraf]

This is another attempt at using HTML files for the _build folder instead of markdown files. Re-doing this because the last PR got stale and has a bunch of merge conflicts. I think that outputting HTML here will keep us less-reliant on a single SSG, and may potentially make it easier to disentangle the SSG and single-page output. It does the following:

• Change the template to use HTML instead of markdown as output (so now the ipynb files will be output to .md in _build)
• Fixes up the CSS to properly select things now that jekyll isn't adding extra CSS when it does a MD->HTML conversion
• Changes the naming of wrappers around the cells to be less-redundant
• Moves the "codeCell" selectors to a separate JS script so that it's only run once per page.

Also, something I realized is that citations actually still work! The output is HTML, but Jekyll will still "process" the HTML like it would a markdown file, so any jekyll shortcodes (which is what we were using for citations) still work! Here's the citations page for this PR:

https://1173-137292803-gh.circle-artifacts.com/0/html/features/citations.html

@emdupre what do you think? We discussed this a while back in #139, any thoughts now, or objections to having HTML be the output here?

[choldgraf]

Here's how the hiding page looks!

https://1173-137292803-gh.circle-artifacts.com/0/html/features/hiding.html

[choldgraf]

ok I'm gonna give this a merge so that we can see if it breaks people's books who are running off of master :-) I just cut a release so we should have some runway to find bugs etc.