Additional keys for 'html-classes' extra.

[toabi]

It would be very useful to accept generic keys instead of only pre and code.

My specific use-case is to append classes to tables to use the Bootstrap CSS styles.

[nicholasserra]

Pull requests welcome :D

[devvyn]

My use case is the same as toabi's.

Also, I noticed the img tag seems to work with html-classes as well as the pre and code tag. Documentation might need an update for that, if it's true.

I made changes that update the docs and the tables and wiki-tables methods.