Add unicodedata module and fixes the default extensions settings.

I have make `markdown` parser work for windows.

1. add the unicodedata 
2. change `"extensions" = "extra"`, to `"extensions" = ["extra"]`

Would please merge it.

And as unicodedata package added I modified the `markdown_extensions.py` in a dirty way. Sorry I'm not a python programmer :( . Would you please help me clean it up.