Add support for source_url for images

[matt-subroot]

Or actually, just total custom childkey to specify what the url key is called. I'm pulling data into a Node backend service from Wordpress, and Wordpress' image urls are labeled "source_url". I don't want to use the gatsby-source-wordpress plugin because I want it all to run through a gateway and backend service first as a proxy. Thus, I am stuck using Wordpress' API and data structures, but they don't fit this plugin's url chilldkey structure.

Unless I am misunderstanding how to use the plugin, this seems way too rigid for a wide range of usage as not everyone is in control of the key names for their images.

[matt-subroot]

My current workaround is to build a transform class in my service that alters the keys from WP, but would like to be able to pass data without creating transforms everywhere

[amcc]

I have the same issue