Migration seeds

[anasbarg]

Add pragma seed command that takes a GraphQL document and executes it.

We also need to add a config seeds which is a path pattern such as ./seeds/*.gql and takes all of these documents and executes them on the first migration (every run in dev mode, and only once in production mode)

[anasbarg]

This is not very important now, because you can solve any problem that this feature solves with super user (root user) access. Checkout #67.