Fixed gist id must be a number and `data-line` not working

[surdu]

The following bugs where fixed:

• data-line attribute not working anymore, probably due to layout change on GitHub's part
• the id of a Gist is not an number only (anymore?). For example, this is a valid Gist id: aa01a6093a52b3fc7f6e91852beb9b69

The following improvements where added:

• when removing the footer, also remove the double bottom border of the code snippet

[surdu]

I also removed this piece of code, as it looks like it did nothing useful. If I'm mistaken, please let me know and I'll revert the change.