Allow set proxy for npm

[LeungJacky]

Currently cannot config the proxy

[oculus42]

This is covered by #237 #459 #570
There are some good comments in those tickets.

It's not a durable fix by any means, but this was my workaround for local testing:

• Open a terminal.
• cd ~/Library/Application Support/runjs (not sure the Windows equivalent
• Create or copy the .npmrc file with the appropriate proxy.
• npm install -S the package(s) from my internal proxy.
• Remove the .npmrc file to avoid messing up RunJS normal package handling.

[lukehaas]

RunJS creates an empty .npmrc file in ~/Library/Application Support/runjs.
You can add whatever config you need to it. RunJS will then use it for installing and searching.