LSP support

[maralla]

Language server protocol support.

Tested languages: go(gopls), rust(rls), c(clangd)

Usage:

• Install a language server, for example gopls for golang.
• Configuration:

let g:completor_filetype_map = {
      \ 'go':   {'ft': 'lsp', 'cmd': 'gopls'},
      \ 'rust': {'ft': 'lsp', 'cmd': 'rls'},
      \ }

The lsp completer is compatible with previous completers, to disable lsp completion just remove the corresponding config in g:completor_filetype_map.