The markdown parser used in StoryBoard (marked.js) is buggy and
mostly unmaintained. This commit replaces it with a better library
which is actually maintained.
There is a small change to functionality incurred with this change,
we no longer automatically attempt to highlight every code block by
guessing the language. Instead only fenced code blocks are
highlighted, and then only if the language is specified. For example:
``` python
def foo():
return 'foo'
```
Change-Id: I8ee562e3e97ab6f0d09a21c27907d1a0959aceb1
980e5d3ec3