Adds support for setting GraphQL Playground endpoints.

[chris-ramon]

Description

• Built on top of: Add ability to configure GraphcoolPlayground endpoint + subscription endpoint #86 — Thanks a lot @AttilaTheFun 👍
• {graphql,graphcoolPlayground.go}: Adds support for setting GraphQL Playground endpoints, eg:

h := handler.New(&handler.Config{
	Schema:     &schema,
	Playground: true,
	PlaygroundConfig: &handler.PlaygroundConfig{
		Endpoint:             "/custom-path/graphql",
		SubscriptionEndpoint: "/custom-path/ws",
	},
})

Test Plan

• Unit tests.

[coveralls]

coverage: 87.864% (+0.4%) from 87.437%
when pulling 5f4c951 on graphql-playground-endpoint-support
into f96ffdd on master.