Added (a very simple) notes liquid tag

So I can add notes while I'm working on a blog post
and they'll be rendered using a skethflow-type font
to distinguish them from the actual blog content.

It is very basic...e.g., it falls down if the note
is more than one paragraph. I'll try it out for a
while and see if it needs extending.

It's purely to highlight work in progress, etc. Things
within a notes tag should never get to the live set.
(I don't think GitHub pages handles custom liquid tags
anyway.)