CSS Tweaks

- Include octicons link icon only and add support for displaying the
anchor link icon
- Explicitly define list styles for each level to ensure all list
levels are displayed proper
- If using the API with mode: “gfm” markdown will be returned with
task-list support; add task list support CSS

It is important to note that the Github API for Markdown for some reason doesn't support task-lists when mode == "markdown".  Yet Github readmes do support static tasklists.

When using mode == "gfm", task-list is supported, but header anchors will not appear.  The API seems to use the commenting systems version of gfm instead of the readme version of gfm.

Just an FYI for people when they see a certain feature supported in one mode but not the other.