nvim: Extend lsp for nvim lua development

Added new functionality to lsp: hovering, go-to definition, referencing,
implementations, etc accessible through the usual hotkeys.

Added commands to invoke LspHover and temporarily disable lsp for the
current buffer (is re-enabled on re-entering the buffer, e.g. with
:e<cr>)

Added tjdevries extended lua lsp implementation containing functionality
for the nvim api itself (especially useful for `vim.api`, `vim.fn`).

nvim/.config/nvim/plugin/nvim-lspconfig.vim

@@ -1,12 +1,6 @@
-lua << END
+command! LspHover lua vim.lsp.buf.hover()<CR>
+command! LspDisable lua vim.lsp.stop_client(vim.lsp.get_active_clients())<CR>
 
-require'nvim_lsp'.pyls.setup{}
-require'nvim_lsp'.vimls.setup{}
-require'nvim_lsp'.bashls.setup{}
-require'nvim_lsp'.gopls.setup{}
-require'nvim_lsp'.texlab.setup{}
-
--- requires manual `:LspInstall sumneko_lua`
-require'nvim_lsp'.sumneko_lua.setup{}
-
-END
+" actual LSP config takes place in lua ('lua/lsp')
+lua require("nvim-lspconfig")
+setlocal omnifunc=v:lua.vim.lsp.omnifunc