Surround sexy comments

[albfan]

With #546 around I want to be able to comment a block of code in less intrusive way possible:

from:

  <metadata>
    <property name="variant">dark</property>
    <property name="light-variant">Adwaita</property>
  </metadata>

to:

  <!--
  <metadata>
    <property name="variant">dark</property>
    <property name="light-variant">Adwaita</property>
  </metadata>
  -->

I did a basic implementation, but probably lacks some needed logic