#39 Use CSS generated content for diff filler

[fritzophrenic]

Look into generated content (in particular the CSS "leader()" function) for generating diff filler: https://www.w3.org/TR/css-content-3/#leaders

In theory this would let us not insert a bunch of fixed-width spaces, we might even fill the entire table cell!

Check browser support; currently too new for a page on caniuse.com. Possibly not widely supported enough to use yet!