Add Proxy-Configuration for business and special environments

[ghost]

Background:
As mentioned in this pull-request, this plugin would be much more powerful with an proxy configuration option. Foremost in business-environments.
I opened this issue so discussions can take place.

What we need:
We use proxies internally, so we need an option to configure this specific proxy for node-fetch

What to do?

• Allow to set "proxyUrl" in the configuration file
• Allow to use "PROXY_URL", "http_proxy", "https_proxy" in the environment vars
• Pass configuration to node-fetch

Alternatives?

• replace node-fetch with node-fetch-with-proxy which supports the HTTP_PROXY env-var

Our Solution:
We can deploy the plugin for our need internally but i wanted to share this feature with the world. We are not in need to see this feature coming officially, but it would be great.