Add workspaces support to `npm init`
- Fixes `npm exec` respecting `script-shell` option value
- Refactored `lib/exec.js` into `libnpmexec`
- Updates init-package-json@2.0.3
- Added ability to create a new workspace using the -w config
PR-URL: https://github.com/npm/cli/pull/3095
Credit: @ruyadorno
Close: #3095
Reviewed-by: @wraithgar
