WIP: lsp manager

[prabirshrestha]

Do not merge. This is WIP. This supersedes #10

Apis aren't locked (lot to clean up here) but here is what it currently supports.

• support serial async control flow via lsp#utils#step#start([....])
• support parallel async control flow via lsp#utils#step#parallel([...])
• ability to register a lsp#register_server

au User lsp_setup call lsp#register_server({
     \ 'name': 'go-langserver',
     \ 'cmd': {server_info->['go-langserver']},
     \ 'whitelist': ['go'],
     \ })

• auto call textDocument/didOpen
• auto call textDocument/didSave
• auto call textDocument/didChange. Currently only calls when leaving insert mode or when saving the file.
• lsp#get_whitelisted_servers() to retrieve active servers for the buffer
• send custom request via lsp#send_request. params and on_notification are optional. automatically flushes the buffer for the server

call lsp#send_request('server_name', {
    \ 'method': 'initialize',
    \ 'params:' {....},
    \ 'on_notification': {server_name, response->lsp#log(server_name, response)},
    \ })

• send custom notifications via lsp#send_notification. params is optional. automatically flushes the buffer for the server.

call lsp#send_notification('server_name', {
    \ 'method': 'initialize',
    \ 'params:' {....},
    \ })

The goal is for the core to be ui independent and then allow ui to be built on top of the core. For example getting document/workspace symbols can be implemented in loclist, quickfix, ctrlp, unite, denite, fzf. Most likely I will only be shipping only quickfix and loclist by default, rest would be as a separate vim plugin.

• :LspGetWorkspaceSymbols
• :LspGetDocumentSymbols

....Lot more todos... will be pushing this to dev branch soon.