add EXTENSION_NO_EMPTY_LINE_BEFORE_BLOCK flag to make it closer to GFM

Hi,
blackfriday is great, but I found [one of the GFM features](https://help.github.com/articles/github-flavored-markdown#newlines) was not supported. Please tell me if I was wrong.


I am trying to make Blackfriday support this feature and commit this patch. 
Hope this helps.
There could be some faults I had never considered ye, any comments are highly appreciated.