External featured image from bing

Adds functionality to search images from post page and attach as featured image. You will require to create an API key on bing for search API. GOTO: https://azure.microsoft.com/en-us/services/cognitive-services/bing-image-search-api/ or https://dev.cognitive.microsoft.com/docs/services/56b43f0ccf5ff8098cef3808/operations/571fab09dbe2d933e891028f And generate your «Ocp-Apim-Subscription-Key»