[Feature request] Add plugin option for prismic.io address

[dturkington49]

When standing up automated testing for my Gatsby project, I need to mock a Prismic GraphQL schema. In order to do this, I need a plugin option which allows the full path of the Prismic repository to be configured. This could be done via an additional plugin option. This option could be used instead of repositoryName and would control the full path (instead of hardcoding https://${repositoryName}.prismic.io). This would enable a mock Prismic GraphQL service to be stood up at a local address and tested against.