The problem with adding so much complexity to the parser is that it's now impossible to predict how any given Markdown is going to render on Github. If Github's parser was open-source, you could at least look at the source to figure out this stuff is handled, but it isn't, so the only alternative is tedious experimentation.

One of the main draws of Markdown was that it was so simple you were rarely left guessing how any given input would be handled (though the lack of standardization hampered that).