Implement post_install option that loads installed starterkit right away

[bmuenzenmeyer]

In order to make editions more user-friendly, it would be nice if an npm postinstall script existed to automatically load the starterkit dist/ into source/.

This could be governed by a config option in patternlab-config.json, or is perhaps not necessary at all.

Doing this would then let us clear out soure/ inside editions and instead include a starterkit in package.json - addressing pattern-lab/edition-node-gulp#34 better