`defaultLinkTo` makes anchors point to the correct path which broke
parsing because it expected href to start with `#gistcomment`.
This commit changes the implementation to make `defaultLinkTo` point
to the correct page (using `getURI` instead of `self::URI`) and search
with `*=` instead of `^=`.
Adds a new bridge for https://gist.github.com
The bridge generates feeds for comments on a particular gist based on
the gist ID or full URI. For better readability the general behavior
of code sections is manually restored with the original CSS styles
from GitHub.
