ApolloServer builds in graphql-playground rather than graphiql, so we no longer
provide middleware for serving GraphiQL.
If this turns out to be an unpopular choice, we can always add support for
graphiql instead of graphql-playground back in later.
We prefer graphql-playground because it allows you to enter HTTP request
headers, view query history, and explicitly supports graphql@0.13.
* feat: ApolloServer is created and exported by all variants
* docs: add initial docs around exporting ApolloServer
* feat: export gql from integrations
* docs: change apollo-server examples to use express with registerServer
* server: remove registerExpressServer
* core, docs: comment functions, fix api reference, context creation more like middleware args
* docs: fix integrationed typo
