Avoid duplicated output using highlight tags

While using Rouge and an `highlight` tag, the output was duplicated
since the `output` variable in the Liquid tag definition was equal to
the highlighter's prefix value and the `<<` method changes its receiver.

Therefore, we should simply define an empty string and append the prefix
if it is present.
Collections' "written" attribute has been changed to "output".

This is a follow-up to 068d921ef649501ffc8cf57f2a3b124c7378a9c5.