Fix build on windows platform

[BurtHarris]

Wow! I've been looking for a template like this. It has more than I could have hoped for. Thank you.

The build scripts in package.json currently don't work on Windows, and the fix is simple. There are well-known npm packages to solve them. cross-env, mkdirp, and rimraf are cross-platform replacements for just such occasions.

I want to contribute back, so I'll try to submit a PR shortly.

[Debdut]

Thanks, a PR will be awesome 🙏🏻

[nascarsayan]

It should now be working on mac, windows and linux platforms.