Add feature for emphasizing some lines in a code block.

A code blocked headed by “:::python{1,3}” now emphasizes the first and third lines. With fences enabled, ```python{1,3} has the same effect.

Pygments has had this feature for a while. Highlighted lines have a yellow background, which is very useful for drawing a reader's attention to specific lines in a long code block. Some other Markdown implementations use this syntax for specifying which lines to highlight, I thought Python-Markdown would benefit from the same feature.