Add support for server specific handlers

[martskins]

This PR adds support for server specific handlers that are specified via config. Other than enabling the user to run certain handlers for a specific server. Becase of this being via config, we can register the handlers before initialization, which ensures all events of that type are handled correctly.

An example configuration to use this would look like this:

let g:LanguageClient_serverCommands = {}
let g:LanguageClient_serverCommands.go =  {
            \ 'name': 'gopls',
            \ 'command': ['gopls'],
                        \ 'handlers': {
                        \  'window/logMessage': 'LogProgress'
                        \ }
            \}

Note that this is only possible with the newly introduced way of specifying a server command.

Closes #1176

[martskins]

@fanzeyi if you could try this one out and see if it solves your issue it would be great. I tested it locally and it seems to work just fine, but maybe I missed something for your use-case.