Supply additional headers with request
Hi there,

First, big props for this lib! Very easy to setup 👍  I am trying to setup a connection with the discogs oauth api, but they require to send an additional user-agent header. How can I configure this in the grant lib? Here's my setup:

``` js
var express = require('express')
var logger = require('morgan')
var session = require('express-session')
var Grant = require('grant-express')

var grant = new Grant({
  'server': {
    protocol: 'http',
    transport: 'session',
    state: true,
  },
  'discogs': {
    key: process.env.DISCOGS_CONSUMER_KEY,
    secret: process.env.DISCOGS_CONSUMER_KEY,
    callback: '/handle_discogs_callback',
  },
})

var app = express()
app.use(logger('dev'))
app.use(session({ name: 'grant', secret: 'very secret', saveUninitialized: false, resave: false }))
app.use(grant)

app.get('/handle_discogs_callback', (req, res) => {
  res.end(JSON.stringify(req.query))
});

app.listen(4000, () => {
  console.log('Express server listening on port ' + 4000)
});
```
