Linked stylesheets should auto-refresh on edit

[mbostock]

If you have

<link rel="stylesheet" type="text/css" href="test.css">

then if you edit test.css, we want the page to update. This means:

1. We need to detect test.css as a file and watch it (which should already be happening in normalizePieceHtml, but doesn’t appear to be working as intended for some reason), and
2. We need to rewrite the href attribute to include the content hash, so that when the linked file changes, the browser will pick up the new file as intended.

[mbostock]

Fixed in #843.