[[_]] float clearing tag experiment

Hiya,
This is trying to address https://github.com/gollum/gollum/issues/653

Automatic clearing on page headers is beyond me, but I did an experiment with a special [[_]] gollum tag to add at the end of text to display, seems to work great so far !

I don't know much about the code base so there may be implications i'm not aware of. Hope this helps !