LSP Find references support

Hi Maralla,

Love your plugin and that fact that you've added basic LSP support to it. Would it be possible to add 'Find references' support as well similar to what is supported by other LSP plugins (https://github.com/prabirshrestha/vim-lsp, https://github.com/autozimu/LanguageClient-neovim). Find references and find definition are the main actions I think most people use in LSP and right now it seems only one of them is supported. Could you support find references as well please.

Thanks.

[maralla]

The support for finding references is implemented in d218114.

Call the function :call completor#do("references") to show the references. You can map a key for convenience.