Add support for extensible CompletionItemKinds and SymbolKinds

[daviwil]

This is a new addition to the v3 protocol, we'll wait for a bit for the API to stabilize before supporting it:

microsoft/language-server-protocol@05e3af3#diff-7327496955357b9792bab8720dc507d6

[damieng]

Waiting for the spec to stabilize around here and some language servers to implement.

[UziTech]

Development of atom-languageclient has officially moved to https://github.com/atom-ide-community/atom-languageclient 🎉

If this is still an issue please consider opening an issue on that repo.