post_url tag raises ArgumentError for invalid name

Using post_url tag with invalid name raises TypeError.

It should raise ArgumentError and should display detail of the error to
fix the error.
added error description for invalid post name
Default Maruku fenced_code_blocks to ON.

Extends https://github.com/jekyll/jekyll/pull/1799
Add two tests for Maruku fenced code blocks
made error description more helpful
variables in include tag with filters
removed duplicate test scenario
edited raise message when validating included file
edited documentation for include tag
Add in 1.4.1 and 1.4.2 changelog
Add `path` to required methods

The error messages in this module call `#path`, but the module does not require the `path` method to be implemented. This change documents that classes which include `Convertible` must define `path`.

See #1846 for discussion.
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?