Fix incorrectly flags rule valid-template-root for pug

fixes vuejs#165

Author: armano2

lib/rules/valid-template-root.js

@@ -31,6 +31,11 @@ function create (context) {
         return
       }
 
+      const lang = utils.getAttribute(element, 'lang')
+      if (lang && lang.value.value !== 'html') {
+        return // Ignore pug and ....
+      }
+
       const hasSrc = utils.hasAttribute(element, 'src')
       const rootElements = []
       let extraText = null

tests/lib/rules/valid-template-root.js

@@ -31,6 +31,10 @@ tester.run('valid-template-root', rule, {
       filename: 'test.vue',
       code: '<template><div>abc</div></template>'
     },
+    {
+      filename: 'test.vue',
+      code: '<template lang="pug"></template>'
+    },
     {
       filename: 'test.vue',
       code: '<template>\n    <div>abc</div>\n</template>'