Code line preview doesn't work with GET parameters

[dgw]

GitHub puts the view mode into the URL sometimes when viewing renderable files (Markdown, reST, etc.), which can break sopel-github parsing the link:

<dgw> https://github.com/Exirel/sopel/blob/34777e2500f713b250413b6d3c978004e04497c3/docs/source/tutorials/configurable.rst?plain=1#L65

(That is, Sopel doesn't recognize that it should handle the link.) Compare to the plugin behavior without ?plain=1:

<dgw> https://github.com/Exirel/sopel/blob/34777e2500f713b250413b6d3c978004e04497c3/docs/source/tutorials/configurable.rst#L65
<Sopel> [GitHub] [Exirel/sopel] docs/source/tutorials/configurable.rst @ 34777e2500f713b250413b6d3c978004e04497c3 | L65: bot.say(f'I want a {fruit}!')