Add unicodedata module and fixes the default extensions settings.

I have make `markdown` parser work for windows.

1. add the unicodedata (800k large when uncompressed.)
2. change `"extensions" = "extra"`, to `"extensions" = ["extra"]`

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.

Would you please consider merge it.