Add gatsby cli for graphiql only

[webrgp]

Summary

When I am in early stage development, connecting to 3rd party APIs, implementing queries, fragments, etc OR having to implement or debug existing queries or fragments, due to API changes, it would be nice to be able to run an command that would basically just run the GraphQL aspect of Gatsby.

Suggested solution

The command could be something like gatsby develop:graphql or just gatsby graphql

$ gatsby develop:graphql
...
...
View GraphiQL, an in-browser IDE, to explore your site's data and schema
⠀
  http://localhost:8000/___graphql
⠀
Note that the development build is not optimized.

I think that would make easier to work on project that interact with big APIs without the clutter of related errors connected to other components, etc, or to implement type extension.

Right now I use a separate gatsby-starter-minimal just to work queries and implement on the project.