[rfc]: added proxy support via proxy-agent

[c33s]

sadly node has not added proxy support to its core yet (nodejs/node#15620) so it is necessary to implement it in each client lib -.-

i added proxy support to via https://github.com/TooTallNate/node-proxy-agent to your awesome lib.

i haven't added tests yes and i am not sure about correctly updating the package-lock.json (had not worked on a node module before). is it just a npm install required to generate the correct lock file or is there something special for a node module?

also i am not quite happy with the mixed type variable (boolean/object), i thought about empty object, object and undefined

references:

• Problem configuring node-fetch with proxy and TLS config(ciphers) node-fetch/node-fetch#1016

edit: not really an idea how to test this.

edit2: as proxyagent should work transparent, maybe using it could be the default