Add note to FAQ about nerdcommenter, and add tests for it

test/test_nerdcommenter.vader

@@ -0,0 +1,40 @@
+Execute (Configure NERDCommenter to support Vue files):
+  let g:ft = ''
+  function! NERDCommenter_before()
+    if &ft == 'vue'
+      let g:ft = 'vue'
+      let stack = synstack(line('.'), col('.'))
+      if len(stack) > 0
+        let syn = synIDattr((stack)[0], 'name')
+        if len(syn) > 0
+          exe 'setf ' . substitute(tolower(syn), '^vue_', '', '')
+        endif
+      endif
+    endif
+  endfunction
+  function! NERDCommenter_after()
+    if g:ft == 'vue'
+      setf vue
+      let g:ft = ''
+    endif
+  endfunction
+
+Given vue:
+  <template>
+    <div></div>
+  </template>
+  <style>
+    .glitters { color: gold }
+  <style>
+
+Do:
+  j\cc
+  3j\cc
+
+Expect:
+  <template>
+    <!--<div></div>-->
+  </template>
+  <style>
+    /*.glitters { color: gold }*/
+  <style>