Exclude links already rendered by GitHub

[fregante]

GitHub implemented this natively as part of https://github.com/features/issues

Some links are still not covered by GitHub so for now the extension is still somewhat useful. I think it should exclude already-rendered links though. The demo page for example isn't covered #57

[kidonng]

FYI, example for ordinary links to issue:

<a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="729085483" data-permission-text="Title is private" data-url="https://github.com/nix-community/home-manager/issues/1571" data-hovercard-type="pull_request" data-hovercard-url="/nix-community/home-manager/pull/1571/hovercard" href="https://github.com/nix-community/home-manager/pull/1571">#1571</a>

vs. rendered links:

<span class="reference"><svg class="octicon octicon-git-merge merged color-fg-done mr-1" title="Merged" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path fill-rule="evenodd" d="M5 3.254V3.25v.005a.75.75 0 110-.005v.004zm.45 1.9a2.25 2.25 0 10-1.95.218v5.256a2.25 2.25 0 101.5 0V7.123A5.735 5.735 0 009.25 9h1.378a2.251 2.251 0 100-1.5H9.25a4.25 4.25 0 01-3.8-2.346zM12.75 9a.75.75 0 100-1.5.75.75 0 000 1.5zm-8.5 4.5a.75.75 0 100-1.5.75.75 0 000 1.5z"></path></svg><a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="729085483" data-permission-text="Title is private" data-url="https://github.com/nix-community/home-manager/issues/1571" data-hovercard-type="pull_request" data-hovercard-url="/nix-community/home-manager/pull/1571/hovercard" href="https://github.com/nix-community/home-manager/pull/1571">README: Remove dangling reference to IcedTea<span class="issue-shorthand"> #1571</span></a></span>

May just exclude links with .reference parents.

[janpio]

The screenshots I posted in #68 (comment) indicate that the GitHub style is slightly different in that the link text itself is not updated - that might still be valuable to have for visual consistency.