Adds GraphQL request handlers

[kettanaito]

• Closes Support mocking GraphQL API #39

Roadmap

• Add the graphql request handlers to the standard request handlers
• Add the graphql.query() request handler
• Expose req.variables to the response resolver
• Set req.variables to an empty object, if no variables are set
• Make context utilities request handler-specific
• Add ctx.errors() utility for mocking GraphQL errors
• Add the graphql.mutation() request handler
• Support RegExp as selector.operation to handle multiple GraphQL operations matching the expression
• Parse the query to determine operation name (don't rely on Apollo, not everybody uses it)
• Add integration tests
  • Query
  • Mutation
  • Variables
  • Errors

[kettanaito]

GraphQL subscriptions will be introduced later, when the necessary research is done on the topic. Query and mutation handling is a meaningful functionality for a developer.