Add helpful comments to GraphiQL queries

[joshprice]

See the example here # Welcome to GraphQL Elixir....

http://graphql-swapi.parseapp.com/

[joshprice]

Done, but could also configure a set of helpful example queries in plug config. Something like:

forward "/api", GraphQL.Plug, schema: {Schema, :schema}, example: """
query A { greeting }
# fragment ...
"""