Add emphasize-lines directive to code blocks

[fvicent]

I suggest to add the emphasize-lines directive to code blocks, Sphinx-like. Emphasizing lines is an already supported feature in Pygments, so the implementation is trivial. I've already patched my local Nikola installation and it has been working fine. I can submit a PR with the proper tests if you are ok with this addition.

This is an example taken from the Sphinx docs of how it works:

.. code-block:: python
   :emphasize-lines: 3,5

   def some_function():
       interesting = False
       print('This line is highlighted.')
       print('This one is not...')
       print('...but this one is.')

Line ranges are also supported, such as:

   :emphasize-lines: 1-3,5-9,15

[ralsina]

If you want to submit the PR that would be great!