gatsby-source-wordpress: add jwt_base_path option

[rotexhawk]

Currently gatsby-source-wordpress hardcodes the jwt token endpoint /jwt-auth/v1/token. I am using https://github.com/jonathan-dejong/simple-jwt-authentication/wiki/Documentation which uses /simple-jwt-authentication/v1/token endpoint instead. It would be great to pass the basepath in jwt options so we can support this plugin and future plugins that use different namespace.

[wardpeet]

I'm referencing gatsbyjs/rfcs#23 as this might be valuable for the RFC.