feat: add gotOptions constructor options

[buschtoens]

This allows the user to pass optional options through to the got instance. For instance, users can now connect via proxies.

Here is an example using caw, as further described in sindresorhus/got#435 and originally brought up in sindresorhus/got#79.

import Cloudflare from 'cloudflare':
import caw from 'caw';

const cf = new Cloudflare({
  email: 'you@example.com',
  key: '<your API key here>',
  gotOptions: {
    agent: caw({ protocol: 'https' }) // Cloudflare API is HTTPS only
  }
});

[coveralls]

Coverage decreased (-0.9%) to 85.714% when pulling 061c9a4 on buschtoens:feat-passing-options-to-got into c3e1c83 on cloudflare:master.

[terinjokes]

I'm probably going to implement this in a different way, since got is an implementation detail (and worse: likely to be removed). Can you open an issue instead?