Use $stderr, not STDERR, $stderr points to STDERR.
New is implied by `raise`, 2nd is the message.
Allow custom Markdown processors.
Depend on Jekyll.logger.error, not $stderr
Merge branch 'steal-envygeeks-custom-markdown-processors' of git://github.com/gjtorikian/jekyll into gjtorikian-steal-envygeeks-custom-markdown-processors

* 'steal-envygeeks-custom-markdown-processors' of git://github.com/gjtorikian/jekyll:
  Depend on Jekyll.logger.error, not $stderr
  Allow custom Markdown processors.
  New is implied by `raise`, 2nd is the message.
  Use $stderr, not STDERR, $stderr points to STDERR.
Use downcase.
Extract checks for acceptable custom markdown processors to method.

We should probably write more about what it does and how it works in a TomDoc block above.
@envygeeks, want to give that a shot?
Fix error in exception message in Markdown Converter [ci skip]
Add TomDoc for Jekyll::Converters::Markdown#allowed_custom_class?
Add docs for custom markdown processors.
Merge pull request #1872 from jekyll/gjtorikian-steal-envygeeks-custom-markdown-processors