Fixed some issues with fenced code blocks.
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).

Kapeli/php-markdown,jockchou/php-markdown,filipstefansson/bootstrap-3-shortcodes,mariokonrad/php-markdown,robbieaverill/php-markdown,filipstefansson/bootstrap-3-shortcodes,MWDelaney/bootstrap-3-shortcodes,n054/php-markdown,ashur/php-markdown,queirozfcom/php-markdown-with-syntax-highlight,MWDelaney/bootstrap-3-shortcodes,MWDelaney/bootstrap-3-shortcodes