[vim] Don't insert chars if key is not bound

Author: adrianheine

keymap/vim.js

@@ -866,7 +866,7 @@
         if (vim.insertMode) { command = handleKeyInsertMode(); }
         else { command = handleKeyNonInsertMode(); }
         if (command === false) {
-          return undefined;
+          return !vim.insertMode && key.length === 1 ? function() { return true; } : undefined;
         } else if (command === true) {
           // TODO: Look into using CodeMirror's multi-key handling.
           // Return no-op since we are caching the key. Counts as handled, but