Add support for npm proxy

Configure npm to use proxies if it is installed.

While the [docs](https://www.npmjs.org/doc/misc/npm-config.html) says they would use to env vars, but the [code](https://github.com/npm/npm/blob/v1.4.3/lib/utils/fetch.js#L76-L79) and issues suggest otherwise.
So we use `npm config set/delete`. And hook also after each provisioner run.

Fixes #50