Add support for Shell Script code in GitHub Markdown

[aboettger]

I have script code in a markdown file.

[rgbkrk]

Now that's an interesting one. Guess we'd have to pick up what's inside the GFM code fence block. We depend on the language of the file right now (in this case, it would detect Markdown and not know what to do).

[erran]

You could always temporarily switch the grammar for what it's worth. In terms of detecting the grammar that gets a bit more complex (but it'd be admitting-ly cool to see).

[danielbayley]

Seems like it would be easy to implement running a shell command based on the selection. Detecting the grammar would obviously be much trickier. Maybe it could just run anything between backticks?