Compatibility with WordPress >5.4 & support byline for featured images

• Added ability to force append byline to featured images (for themes without supporting such feature). It is option which can be enable/disable via new option "Featured image byline".
• Fixed for Gutenberg image block in WordPress >5.4.
• Serious improvement of injecting credits for Gutengerg image blocks, including support for customized captions.

Clone of "Image and Media Byline Credits" v1.0.0 by DevBackroom

Clone of "Image and Media Byline Credits" v1.0.0 by DevBackroom taken from here.