Discuss merging redesign

Hi @rhythmus 
This is a dummy pull-req I'm opening to myself, although I'm not yet able to merge it as is.

This looks gorgeous!
And the amount of work you did in one day is unbelievable.  It would take me longer to even read through your changes :-)

I'm going to be quite busy in the next 3 weeks (moving countries, paper deadline) but I absolutely want to merge parts of this...
A few blockers I immediately see:
 - Nested lists don't work.
 - Some formatting is too subtle, barely visible, notably quotes levels are almost indistinguishable.
   I'd like to experiment with changing formatting opacity from well visible during edit mode to faded out when clicking or just moving mouse outside (cf. https://hackpad.com/Introducing-Reading-mode-OI5XL2pSgjk).
 - It's harder to see the source as-is, e.g. how much is a given line is indented?  (A lot of this is because stuff like absolute positioning broke codemirror cursor position.)
    Clear view of the source is IMHO crucial, especially for people who're new to markdown.

I've commented on some things in http://rhythmus.be/mathdown/?doc=DCJtdsxteYC#Mathdown-Bugs
I don't think CM is as bad as you describe, but we'll have to try other approaches.  E.g. moving things to margins may be doable simply by giving bigger padding-left to normal text. 

P.S. note that storing the documents in firebase means that http://mathdown.net/?doc=about and http://rhythmus.be/mathdown/?doc=about edit the very same document.  (Which IMHO is really cool for open source services, one can fork and change the UI without any export-import dance...)
So I changed the description back to refer to "New document button (top right)" so (the rare) visitors to mathdown.net won't be confused by "file icon" reference.
(I should add a view mode taking docs like the landing page and Help from the source; allowing anybody to edit them is sometimes amusing but won't scale...)

Re font choice (as you are an expert): would you recommend Computer Modern as main editing font, or was it just proof-of-concept?  What about users who don't need math and never saw LaTeX?  What do you think of other fonts mathjax 2.3 supports e.g. Neo Euler?