Fix crash when using Plain Text (#129)

lib/constants.js

@@ -85,7 +85,8 @@ export const LANGUAGES = [
     mode: 'auto'
   },
   {
-    name: 'Plain Text'
+    name: 'Plain Text',
+    mode: 'text'
   },
   {
     name: 'C',
@@ -363,7 +364,9 @@ const unfold = (f, seed) => {
 if (typeof window !== 'undefined' && typeof window.navigator !== 'undefined') {
   const alreadyLoaded = new Set()
 
-  LANGUAGES.filter(language => language.mode !== 'auto').forEach(language => {
+  LANGUAGES.filter(
+    language => language.mode !== 'auto' && language.mode !== 'text'
+  ).forEach(language => {
     if (language.mode && !alreadyLoaded.has(language.mode)) {
       alreadyLoaded.add(language.mode)
       language.custom