Improve the syntax of {% footnotes %} block

Prior to this commit, it was impossible to use typography like
parentheses or quotes in the footnote body, as Liquid attempted to
parse it on its own. Now it is possible, as the footnote body no longer
resides lexically inside a Liquid tag.
Update README.markdown
Fallback to use params if there's no tokens.shift

- This is an attempt to keep the minimum of backwards
compatibility. Since the new syntax is much better it
should be the default.