background-image for every block with background-color

[tkapler]

related to #192 but a slighlty different approach, as discussed here https://www.facebook.com/groups/editorskit/permalink/1876390995840066/

possibility to setup also background image for the block (all block where is background color option). So that i would have an option to select color and optionaly also img from media library and its URL (optimaly with selection of the sizes available) would be then added to the style of the block (style="background-image: url(the_url.jpg)") plus there would also be another class for the block - e.g. .has-background-image nowdays it is impossible to do, i have to give block some class and then go to CSS and define the image there, or do some other hacks like taking the image url from the class field etc. via filter ...