switch to redcarpet for markdown

Redcarpet has some interesting extensions, particularly the automatic
adding of anchors to headers for easy intra-article references.
fix interpretation of underscores in include_code

With some setups, mardkown processes the contents of Liquid tags before
the Liquid tags themselves are processed. When the tag name and
parameters contain underscores, this can lead to erros when the
underscores are replaced with html markup (ususaly <em>, <u>, or <i>).

This commit fixes this behavior in include_code with two small changes:
* create an alias for include_code without an underscore (includecode)
* run a gsub on the tag paramters (markup) to strip out html tags
  - `/<\/?\w+>/`, which strips any opening and closing basic tags
    + `<em>`, `</em>`, `<arbitrary>` will be stripped
    + `<tagname param="value">` won't be stripped
minor changes to code figure style