Request: add a type argument to ImageGallery so that we can use whatever props we want for ImageObject

[nathantew14]

When rendering custom components with renderCustomImage and the other similar render props, I often want to pass in additional data for the custom component to work, beyond just showing the image. For example, showing a button conditionally. It still works as long as the objects passed to the images props have the properties I want, but typescript complains:

Property 'createdBy' does not exist on type 'ImageObject'.

I tried to do it myself to make a PR, but unfortunately I'm still too newb at typescript...

If this isn't added by the time I figure it out, I'll make a PR for it :P