Fix liquid_tags for Markdown 2.5

This patch closes issue #312 as far as I can see.

Markdown 2.5 changed the config system for markdown extensions. Inspired by
the render_math plugin, I added code to cope with both versions. New settings
used in the markdown part must go in LT_CONFIG and LT_HELP in `mdx_liquid_tags.py`.

Not Python 3 compatible, uses dict.iteritems()