Add graphql and graphql-tools as a peerDependency.

[ghost]

Currently I'm trying to use graphql-tools. But the graphql packaged in express-graphql and this package seems to conflict and this is the error I'm getting.

{
  "errors": [
    {
      "message": "Schema must be an instance of GraphQLSchema. Also ensure that there are not multiple versions of GraphQL installed in your node_modules directory.",
      "stack": "Error: Schema must be an instance of GraphQLSchema. Also ensure that there are not multiple versions of GraphQL installed in your node_modules directory.\n    at invariant (/home/pyros2097/Code/mine/cress/node_modules/graphql/jsutils/invariant.js:19:11)\n    at validate (/home/pyros2097/Code/mine/cress/node_modules/graphql/validation/validate.js:60:72)\n    at /home/pyros2097/Code/mine/cress/packages/cress-cli/node_modules/express-graphql/dist/index.js:140:52\n    at <anonymous>\n    at process._tickCallback (internal/process/next_tick.js:188:7)"
    }
  ]
}

So I think making this to peerDependency might help.