Incorrect fenced code blocks in github_markdown writer?

[wcaleb]

Should the github_markdown writer be changed to bring it into closer alignment with Github Flavored Markdown documentation and the Github Pages Jekyll parsers?

Writing to github_markdown currently renders fenced code blocks with language attributes like this:

 ``` {.python}
 print 'Hello world'
 ```

Blocks like this do appear to be rendered correctly by GitHub's markdown previewer. But the spec (if it can be called such) for Github Flavored Markdown instead seems to prefer Pandoc's short form for fenced code blocks:

``` python
print 'Hello world'
```

The markdown parsers that support Github Pages (redcarpet and kramdown) also prefer the syntax without brackets. Neither of them recognizes the one that Pandoc currently outputs with github_markdown. Redcarpet (at least the one currently on Github Pages) appears to recognize the short form syntax with either tildes (as produced currently by Pandoc's markdown_phpextra writer) or back ticks.