Introduce zero-dependency Markdown Editor

Dokku: http://markdown.dokku.hypothes.is/
This is an alternative approach to https://github.com/hypothesis/h/pull/1478 
_(Let's try to keep discussion over there.)_

It introduces a simple Markdown editor to help users with no knowledge of Markdown easily insert links, make lists, etc.
![hypothesis](https://cloud.githubusercontent.com/assets/521978/4194840/3bbabd48-37b4-11e4-8cab-cf2cae7b25ad.jpg)

### Features
+ Extremely light weight, there are no extra dependencies added. 
+ Buttons for bold, italics, link, image, quote, code, lists, numbered lists, and LaTex or MathML.
+ Keyboard shortcuts for bold, italics, and link.
+ The buttons are pretty smart. For example, if you select some text and press bold, it will insert the markup around the text you selected. Try them out!


Check that again? The user-select mixin is real, but I don't think that's
the right module to import.
On Sep 23, 2014 2:07 AM, "Aron Carroll" <notifications@github.com> wrote:

> In h/static/styles/markdown-editor.scss:
>
> > +
> > +  .markdown-tools {
> > +    margin-top: 10px;
> > +    border-top: 1px solid #D3D3D3;
> > +    border-left: 1px solid #D3D3D3;
> > +    border-right: 1px solid #D3D3D3;
> > +    border-radius: 2px 2px 0 0;
> > +    width: 100%;
> > +    margin-bottom: -1px;
> > +    padding: .7em .7em .7em .5em;
> > +    user-select: none;
> > +    -webkit-touch-callout: none;
> > +    -webkit-user-select: none;
> > +    -khtml-user-select: none;
> > +    -moz-user-select: none;
> > +    -ms-user-select: none;
>
> Add @import 'compass/layout/stretching'; to the top of the file, then we
> can use @include user-select(none); here.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/hypothesis/h/pull/1479/files#r17898005>.
>