Add support for custom image directory to gatsby-source-wordpress

[brandensilva]

This ones for @TylerBarnes and friends but currently gatsby source wordpress looks for images at wp-content/uploads to use gatsby static images.

This works fine for quite a few wordpress sites but we use Trellis/Roots/Sage to manage a lot of the back-end and they default to app/uploads as their image directory structure.

Add an option for us to specify custom urls for image target lookup so we can take advantage of gatsby's image utilization.