Add support for request-received middleware

[niftylettuce]

The request-received middleware at https://github.com/cabinjs/request-received exposes a Symbol.for property, which this package could conditionally consume if interested (as opposed to just generating a new Date().

The code at https://github.com/expressjs/morgan/blob/master/index.js#L506-L507 would change as follows:

-  this._startAt = process.hrtime()
+  this._startAt = this[Symbol.for('request-received.startAt')] ? this[Symbol.for('request-received.startAt')] : process.hrtime()
-  this._startTime = new Date()
+  this._startTime = this[Symbol.for('request-received.startTime')] ? this[Symbol.for('request-received.startTime')] : new Date()

[niftylettuce]

As similar to expressjs/response-time#19 (comment), note that Symbol is not supported in Node v0.8 and v0.10, only v0.12+.

[niftylettuce]

See #201