Updated GitHub Markdown CSS for latest GitHub changes (ST3)

This is just the same commit from revolunet/sublimetext-markdown-preview#106 , cherry-picked into the ST3 branch.

For reference, here is that pull request's text:

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I noticed that the included CSS stylesheet wasn't fully updated with the current format of the GitHub markdown parser. I've revamped it to more closely match GitHub's current styles, pulling the CSS from GitHub's own CSS when possible.

One improvement of note is that the two image resources GitHub uses in their Markdown output (the checkered line in horizontal rules, and the link/anchor icon next to headlines) have both been embedded directly in the CSS. This allows the images to be displayed without the need for an external  resource, so the generated html is completely portable and doesn't require an internet connection.

For the link icon, I even took care to extract the SVG path from GitHub's octicon font set (where the icon is usually pulled from) and embedded it in the CSS. By keeping it a vector graphic, it will continue to look good at all page sizes.

I also cherry-picked the change into the ST3 branch, while I'll send a separate pull request for.