Added "otherwise" function for chaining possible html elements to use for the same attribute. Added example and updated README.

Hi Brian! I made this because I needed it for the particular thing I'm building here at Seen. I also tested it with your hackernews example to make sure it worked. The only thing I would pay attention to is that it returns None in case a parameter was not found or empty, I'm not sure if that's the same behavior of the original library (I remember sometimes it might have returned some empty strings).

No problems up to now, but I'm going to keep testing it during the upcoming days too.