Deterministic footnote anchors.

Previously we'd been using `read-markdown`'s default footnote prefix,
which comes from `gensym`. As a result this was "random" and could
vary for the same post from one markdown production to the
next.

Instead use a prefix based on the post file name. This is also
guaranteed to be unique, since all the files live in _src/posts/, but
will be deterministic not "random". Two runs of Frog on the same
sources will produce exactly the same output (at least for footnote
anchors). This is good for If-Not-Modified for pages and especially
for Atom and XML feeds.

(Why is there a "footnote prefix" concept, at all? Because the HTML
from multiple posts can be mixed on the same page -- like an index
page. As a result, the anchors for "footnote1" would conflict without
some prefix to "namespace" them.)