Leverage npx to do 'npm install' w/o sudo

[LazarusX]

npx is a tool bundled with npm 5.2 or later (npm 5.2 is released on 2017-07-05, and bundled with Node.js 8.2.0).

In short, npx installs things using ~/.npm/_npx as a "global prefix", and then adds ~/.npm/_npx/bin to $PATH. By leveraging npx, we can avoid using sudo on *nix systems to install the Node.js module template, and combine two commands (npm install -g and yo azure-iot-edge-module) into a single one.

Learn more about npx:

• https://medium.com/@maybekatz/introducing-npx-an-npm-package-runner-55f7d4bd282b
• https://twitter.com/maybekatz/status/883846850171641856

My current work-in-progress branch: https://github.com/Microsoft/vscode-azure-iot-edge/tree/ray-npx

[LazarusX]

Just took time a measure, it took a minute to add a new Node.js module w/ template installation vs. 7 seconds w/o template installation

[LazarusX]

Implemented by #329