Implement proxy feature for HMR support on other plattforms

[mikaelengstrom]

Would love to have a proxy-feature with HMR. So that you could proxy an external system.

Here is an working example

Not sure about the interface, but maybe we could add a settings for remote-proxy-url in .frontendrc? And maybe if it should be overridable from commandline. E.g.
npm run start:proxy --proxy-target=https://how-would-this-look-on-prod.com