Add peeks for idents

[Daniil159x]

Hi!
I found the project very useful. And my usual workflow looks like that: click on some ident and open N new tabs for reference/definition. But in other code view apps I can "peek" source (e. g. https://code.visualstudio.com/docs/editor/editingevolved#_peek, or github).
I have added this feature in single page and dynamic popup:

The changes bring performance impact, but i hope caching handles it.
You can try it on my instance: https://elixir.ravin.gs.

[tleb]

Thanks for that rather consequent feature! I'll be able to give feedback and discuss next week.

You might have seen we faced some annoying performance issues recently (#365), so we'll have to let it rest a bit to add features that worsen performance.