Closes issue #312
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`.
Merge pull request #321 from jcdubacq/issue312

Fix liquid_tags for Markdown 2.5