using a custom http.Agent (extend documentation)

I was searching for a solution to use a custom http.Agent for every request. In early 3.x releases, it was possible to do something like (ref: #1130):
```javascript
const http = require('http')
const request = require('superagent')

const myAgent = new http.Agent()

request(method, url).agent(myAgent)
```

With the PR #1302 the `.agent()` method only accepts some options, but no http.Agent instance.

My solution was to inject a middleware function with `.use(...)`.
Is there any better solution for this scenario?
I would suggest to extend the documentation about using custom http.Agent and `superagent.agent().use()`.

My solution:
```javascript
const myAgent = new http.Agent()
const request = superagent.agent().use(req => req.agent(myAgent))
```