Add fallback to NPM proxy configuration when other proxy configuration is unavailable

[flotwig]

Add support for the proxy and https-proxy configuration variables in NPM to control Cypress's proxy configuration.

Users should still be able to do HTTP_PROXY= (empty string) to disable proxy usage altogether.

This should allow Cypress to "just work" when launched by proxy users who use NPM as a launcher.

This should also be added to the CLI so that it can use npm_config_proxy at npm install time.

[cypress-bot]

The code for this is done in cypress-io/cypress#4705, but has yet to be released.
We'll update this issue and reference the changelog when it's released.

[cypress-bot]

Released in 3.4.1.