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.
Support syntax for highlighted lines like: ```python hl_lines=“1 3”
Allow single as well as double quotes for hl_lines.
Document hl_lines feature for fenced_code and codehilite extensions.
Merge pull request #274 from ajdavis/highlight-lines

Add feature for emphasizing some lines in a code block.