Possibility to set proxy / agent? #5

With node-fetch it's possible to specify a proxy.

// npm install node-fetch
// npm install https-proxy-agent

const fetch = require('node-fetch');
const HttpsProxyAgent = require('https-proxy-agent');


(async () => {
    const proxyAgent = new HttpsProxyAgent('http://46.250.171.31:8080');
    const response = await fetch('https://httpbin.org/ip?json', { agent: proxyAgent});
    const body = await response.text();
    console.log(body);
})();

It would be great if we could specify the fetch options when using the node-loader.

In our org, we have a docker container that globally sets a proxy in the environment, which we only use for a select few calls (which are used inside node-loader-http), but other than that we don't use this proxy. If we were able to configure the proxy for node-loader-http we'd be able to disable proxy on a global level.

I wouldn't mind making a PR to address this, just wondering if it's OK that I go ahead and if there's any possible pitfall that I may need to consider.