Build scripts are not cross-platform

[reduckted]

The build scripts don't work on Windows. This makes it difficult for me (and others) to contribute to this project.

Some examples:

> yarn test
$ rimraf dist && tsc && cp -r test/fixtures dist/test && mocha dist/test --recursive
'cp' is not recognized as an internal or external command,
operable program or batch file.
error Command failed with exit code 1.

> yarn run prepare:package
$ cat package.json | ts-node build/package.ts > dist/package.json
'cat' is not recognized as an internal or external command,
operable program or batch file.
error Command failed with exit code 255.

It would be great if these scripts were cross-platform!

[mgechev]

@reduckted I'll fix this as part of the next release. Will add it to my schedule for next week.