Additional issue/PR event link handling

[half-duplex]

Re: IRC discussion
The utility of issue display can be improved by parsing the "hash" part of URLs and displaying the linked comment instead of only the original issue/PR description.

Current:

<+dgw> Edited to add: "Meh." :P https://github.com/sopel-irc/sopel-github/pull/26#pullrequestreview-226279842                    
< Sopel> [GitHub] [sopel-irc/sopel-github #26] HumorBaby: webhook: restrict incoming requests | A bad actor can use this [snip multi-line description]

An improvement would be something like:

< Sopel> [GitHub] [sopel-irc/sopel-github #26] HumorBaby: webhook: restrict incoming requests | dgw comments: Let's call it ready. Maybe this will go into 0.2.0 after all! haha

[dgw]

Thanks for opening this!

Let's collect known document fragment types and what they map to (for API calls):

• #pullrequestreview-<review_id> — a pull request review, with its review_id (API doc)
• #issuecomment-<comment_id> — a comment on the issue/PR, with its comment_id (API doc; already handled, seemingly since at least v0.1.1)
• #discussion_r<comment_id> — PR file/line comment, with its comment_id (API doc)
• #issue-<comment_id> — ignore; points to the issue/PR description (top comment) we already fetch

Once we figure out what to handle, the list above can be made into a task list in the issue OP.

Are there other timeline events that would be worth parsing permalinks for? Force-pushes? Review requests? Assignments? Milestone changes? Labels? References to other PRs/issues? (Many of them just use a generic #event-<ID> fragment format, where the ID maps to this API call.)

[dgw]

I feel bad about punting this again, but I'd really like to get 0.4.0 out in the next week-ish and it would take longer than that to get everything here squared away.