added "open from web page"

Hello,

After having copy/pasted too many levels from reddit, I decided I could benefit from a "open level from web page" feature in sixcells.
It still needs some minor fixes which I will try to address soon (using the correct charset when loading web page + a better regex for finding levels inside the webpage) but, basically, it works.

And also thanks for this hexcells editor!

Regards,
Olivier