Added centered image and figure text option.

I have had use for centered images using jemdoc, so I added a feature to my fork. Please consider to merge this into yours, as I image more people will find use for it.

In stead of having the rest of the block's content to the right of the image, it is now centered below the image as a figure text.

Best regards,
Søren Bøgeskov Nørgaard