Optional link step (for CI builds)
For CI builds, it would be interesting to make the `skpm-link` step in the `postinstall`script optional (or maybe have it do nothing when it detects certain environment variables?) so that the build process doesn't stop waiting for someone to answer this prompt:

```
The Sketch developer mode is not enabled (http://developer.sketchapp.com/introduction/preferences/#always-reload-scripts-before-running).
Do you want to enable it? (y/N)
```
