Add variations on start and build scripts for Windows systems

[mint-thompson]

These scripts set an environment variable before invoking react-app-rewired. The syntax for doing this is different on Windows systems. Two new scripts are defined to make these actions available to Windows users.

[jafeltra]

I tried using start:win on my Mac and it does not work. So I guess we can discuss whether we want to add cross-env and use that in one script or keep both.

[cmoesel]

I say let's use cross-env. It's designed exactly for this purpose and is super easy to use.

[cmoesel]

I had some time, so I switched this over to using cross-env. It still works on Mac. At this point, we can only assume it works on Windows (if there is truth in advertising).

[cmoesel]

Wow, Mint and Chris. This looks great!