Add link to issue/PR output when triggered by numeric reference

[dgw]

That is, at present there's no way to get to the GitHub page if someone brings up an issue or PR by mentioning its number in a channel where that option is enabled.

<cottongin> #2014
<Sopel> [GitHub] [sopel-irc/sopel #2014] cottongin: find: format replacement text instead
        of "meant" | This simply applies the bold formatting to whatever the new replacement
        text is rather than the "meant" text. […]

Should have been added along with #65 but oh well.