Config for default image in open graph

It is awesome to load first image in the page, but some pages may not have images. Hence I created the "opengraph_default_image" setting and push into the array of images in case there is no images.
Merge pull request #1 from mauris/patch-1

added default image configuration if no image is on site as fallback
Merge pull request #1 from ahmet2106/master

upstream merges