When using html pre/code tags to wrap github's fenced code block syntax,
showdown would parsed them instead of treating them like plain code.
Closes#229
Fix ghCodeBlocks not being correctly parsed inside lists. Also, as a side
effect, fixes issues with consecutive lists and extra paragraphs being
added into lists.
Closes#142, Closes#183, Closes#184
GFM support fenced codeblocks. Showdown, since very early, adopted this too.
It is now possible to disable GFM codeblocks with the option "ghCodeBlocks" set to false.
It is enabled by default