[neovim] Give key bindings a little love; add GUI bindings for neovide

- Clean up the init interface. Move all the init methods into a single
  init_all() call.
- Add two keybindings for <M-j> and <M-k> to move by soft-wrapped lines in
  Normal mode
- Add a few key bindings for the usual shortcuts for cut/copy/paste when in GUI
  mode. In vim, D is the character that represents the Super/Apple key. So,
  <D-x>, <D-c>, and <D-v> now do what you'd expect.

config/nvim/lua/gui.lua

@@ -5,10 +5,13 @@ local function _init_neovide()
         return
     end
 
+    -- No use for these animations.
     vim.g.neovide_cursor_animation_length = 0
     vim.g.neovide_position_animation_length = 0
     vim.g.neovide_scroll_animation_length = 0
 
+    vim.g.neovide_input_macos_option_key_is_meta = "both"
+
     vim.cmd [[ colorscheme lunaperche ]]
 end