[suggestion] allow headers and thumbnails to be set for each contenttype

I can imagine enough reasons why you would want to share {ct} foo but not {ct} bar and I can also imagine allowing varying thumbnail sizes for each {ct} to have more control over bandwidth use.

If for example I don't want people to implement my json as they please for content that is part of my USP, while stripping off my credits and origin. However I do want people to share lists of my products and my offers and my events so that my products and brand gain more exposure.

For thumbnails I may want to change thumbnail size individually to manage my bandwidth a little. This way I could limit the size of content that is shared and viewed a lot while not making it needlessly smaller for content that is viewed and shared less.