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!


This function is misnamed, it's also doing too many things. I'd split this into a few functions.

1) `applyMarkup(markup, placeholder)` that checks for a current selection, then calls either `wrapSelection` or `insertMarkup` depending on if there's a selection. 
2) `wrapSelection(selection, markup)` that takes a selection object checks to see if the markup has been applied. It then calls `insertMarkup`.
3) `insertMarkup(selection, markup, content)` that performs the actual insertion.

Then `insertBold` etc can just call `applyMarkup`