When using `proxyTimeout` it's very difficult to tell the difference
between a regular socket hangup and a timeout because, in both cases, an
`ECONNRESET` error is thrown. Ideally we should be able to identify when
a proxy request has failed because it took too long, this would allow us
to do things like send appropriate `504` status code.

Suddenly throwing a different error would probably be considered a
breaking change because it's possible that users of http-proxy are
relying on the `ECONNRESET` error. I decided to add the custom timeout
error behind a new option for now so that people can opt into using it.

If you set this option:

```js
var proxy = httpProxy.createProxyServer({
  target: 'http://example.com',
  proxyTimeout: 100,
  proxyTimeoutCustomError: true
});
```

Then the error that gets thrown will have a message of `"The proxy
request timed out"` and a code of `ETIMEDOUT` to match Node.js:
https://nodejs.org/api/errors.html#common-system-errors

This allows for custom error handling code like this:

```js
proxy.on('error', function(err, req, res) {
  if (err.code === 'ETIMEDOUT') {
    res.writeHead(504);
  } else {
    res.writeHead(503);
  }
  // ...
});
```

Resolves http-party#1331.