Show diagnostics for all lines in lsp-ui-sideline

[zjp]

I really like how neovim's lsp-ui package displays all diagnostics for all lines, and I'd like to see that behavior in lsp-ui-sideline. I'm halfway through hacking it into my local copy of lsp-ui, so I thought I'd open an issue here and see if this is something you'd be interested in supporting so I can either open a PR when it's done or reorganize my work around monkeypatching lsp-ui in my init config. Let me know!