Add user script option

[milhnl]

• [x] I have read this document: https://miniflux.app/opinionated.html#feature-request

Hey developers of miniflux. First: Thanks for developing this amazing RSS Reader!

I'd like to ask whether a user script option (like the user style option) is something you'd be interested in. I'm currently developing it in a local fork, and don't want to just do a drive-by PR. One of the usecases for me is marking entries in Twitter-like feeds read as soon as I scroll by. I'd rather not permanently fork it, and think having the option to load a script might benefit others. I understand that JS API stability is probably not something you would want to promise, but there's still a lot of things that can be done without it.

Thanks!