Add environment variable for webpack dev HTTPS

[karlhorky]

In our development environment we need to run the webpack asset server with HTTPS.

Would you be open to accepting a pull request to allow this to be configured in Electrode?

Run in https mode like this:

WEBPACK_DEV_HTTPS=true npm start

or add to your package.json:

{
  "scripts": {
    "start": "if test \"$NODE_ENV\" = \"production\"; then npm run prod; else WEBPACK_DEV_HTTPS=true gulp dev; fi"
  }
}

cc @FreaKzero