GFM Enhancements (Backticks)

- Adding support for GFM backticks ``` (for fenced code blocks); in additional to ~~~ already supported in Markdown Extra.
- Adding support for GFM language marker; e.g. ```php
Conflicts:
	markdown.php
Improving GFM-Style Fenced Code Blocks; and Updating to `data-lang=""`.
GFM-Style Fenced Code Blocks Only (no lang attribute)

Removing the additional code that I applied previously to implement the `data-lang` attribute. In retrospect, this was a bad idea; as the existing implementation does a good job with this already when used properly.

Many thanks to Michel Fortin for his help in accomplishing these changes :-) I hope this might be merged into the official copy at some point in the future.
Conflicts:
	markdown.php
Fixed some issues with fenced code blocks.

HTML block parser was confusing backtick fenced code blocks with
code spans, which has no consequence most of the time but could
be problematic if you're writing code spans inside of the code block.
Code span markers are now matched only when backtick fence matching
has failed.

Fixed an issue were two consecutive fenced code blocks with no
blank line between them which was causing the next paragraph to
become a code block (with both tilde and backtick fences).