[Proposal] Split into multiple middlewares

Just a idea, but I think we should embrace express middleware system.

Maybe something like below:

import { parser, validate, execute, errorHandler, graphiql } from 'express-graphql';
import logger from 'graphql-third-party-query-param-logger';

const app = express();

app.use('/graphql', parser(...));
app.use('/graphql', logger(...));
app.use('/graphql', validate(...));
app.use('/graphql', execute(...));
app.use('/graphql', errorHander(...));
app.use('/graphql', graphiql(...));

app.listen(3000);

And then we can use middlewares at any points to log and measure performance, even have more control on input, output and errors.

Eventually, more reasonable, extendable middlewares will born and enrich the whole graphql ecosystem.

Relative issue: #101 #102 #107