All CLA requirements met.

@darkbasic I would like to ask you to provide this as a protocol extension like described in https://github.com/microsoft/language-server-protocol/blob/master/contributing.md. This has the advantage that we have separate files which eases the code review. We could then even have it in a next version of the LSP libraries.

You could take protocol/src/protocol.configuration.ts client/src/configuration.ts and server/src/configuration.ts as a blue print since it sends a request from the server to the client as well.

@dbaeumer thanks, it's under its own standalone proposal now.

Client capabilities are now declared.
@rcjsuen I thinked about it and there is nothing the server can do if the client fails to open the editor. In fact I decided to not return a boolean anymore: if there is an error it can still be catched thanks to the protocol capabilities.

@darkbasic I re-implemented this since a lot has changed since the creation of the PR. I will still mention you in the N&N since you are the original creator of the code.