support github style highlight in url option

[soonhokong]

Examples:

• https://github.com/leanprover/lean/blob/master/library/logic/examples/nuprl_examples.lean#L22-L29
• https://github.com/leanprover/lean/blob/master/library/logic/examples/nuprl_examples.lean#L22
• https://gist.github.com/soonhokong/d57051ec2a929c05833b#file-small_exercises-lean-L27-L30

TODOs:

• [ ] Highlight selected lines
• [ ] Scroll to the first highlighted line.