bug: development on Windows

# Is this a feature request or a bug?
bug

# Expected behavior:
`npm install` just works

# Actual behavior:
postinstall scripts fail on Windows platform.

I know Windows is not very friendly platform for web-development, but `ds` is supposed to be cross-platform tool, so it would be nice have development env running there as well.

I suggest to replace direct bash command with npm package (like [loop](https://www.npmjs.com/package/loop)) or script which will execute `npm i` for desired directories.

# Environment info (where relevant)
- Browser: n/a
- Operating System & Version: Windows any
- Version of Node (`node -v`): v12.4.0
- Version of ds (`ds --version`): 1.0.0