Require at least 1.3.0 for Rouge

Rouge 1.3.0 introduced a `rouge_formatter` helper which is handy to
overwrite the formatter default when using the Redcarpet plugin so let's
require this version at the very least.

An abort statement will be thrown when the installed version is not
correct.
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.
Fix quick site bugs.
Merge pull request #1872 from jekyll/gjtorikian-steal-envygeeks-custom-markdown-processors
Update history to reflect merge of #1872
Merge pull request #1866 from colindean/patch-2
Update history to reflect merge of #1866
Merge pull request #1853 from jekyll/update-history-with-v1.4.x
Merge pull request #1841 from jens-na/include-variable-liquid-filters
Update history to reflect merge of #1841
Merge pull request #1831 from jekyll/default-maruku-fenced-code-blocks