Apply grey background on code-blocks

[Malabarba]

It would be nice to apply a different background to code-blocks in question-mode, just like the style of the website.

I haven't done this yet because there are some minor dificulties:

• The code-block is under an overlay with the sx-question-mode-content face, which has a background. Because overlays take priority over text-properties, any background applied on the code-block as a text-property won't display.
• The code-block is built in a temp buffer and returned as buffer-string, so applying an overlay at this point has no effect (it isn't returned along). The overlay will need to be applied later.

I'm leaving this as a wishlist.