`info`: support user-extensible documentation providers about cider-nrepl

[vemv]

@bbatsov @alexander-yakushev feel free to review the idea :)

Seems a small task, would be happy to get it right from the start regardless.

[alexander-yakushev]

Sounds like a quite useful feature!

[vemv]

Thanks!

I reckon that an alternative approach would be to use defsource.

But we've had troudbles in the past getting people to correctly wire up defsource to our mranderson-ized Compliment.

It also can be considered compelling for lib authors to code just an .edn and one or two defns, without having them pull a dependency, interact with it, etc.

I would however foster defsource for authors wanting to offer custom completions for their libraries.

But I'd see first if the simplest thing (doc only, dep-free) does get acceptance.