New "scripts" property to setup global pre/post scripts

[Matthew Kelly]

Packages each have their own pre/post install/uninstall script properties, as well as each provider - soon. This is to create a global one for running a pre-install script before all installations begins (etc.):

---
scripts:
  pre:
    install:
    uninstall:
  post:
    install:
    uninstall:

packages:
- name: etc

This can just re-use the same [ParcelScripts] class like packages.