Feature: Run scripts / callback after templating is complete.

[ryan-roemer]

Originally from https://github.com/FormidableLabs/builder-init/issues/12

Research and add a scripts (or somethign) functionality for things like having a project after initialization do:

$ npm install
$ node_modules/.bin/builder run check-ci

Task:

• [ ] Research scripts use cases and implement.
• [ ] Colored logging output.

/cc @kenwheeler

[kenwheeler]

I would like the ability to:

• Run a post run script like cd {dir} && yarn
• Post run output like Wowee, you're all set! cd into your new app and run yarn and then npm run dev