WIP: Add script hooks

[jlowellwofford]

This PR adds the ability to specify (through various means) uinit-style scripts to be run at:

• Container creation (outside container namespace)
• Container init (inside container namespace)
• Container exit (outside container namespace)
• Container delete (outside container namespace)

Scripts can be provided as:

• path to a file in the root filesystem
• path to a file relative to image root
• raw json-encoded multi-line plain text (ugly!)
• base64 encoded plain text
• gzipped + base64 encoded
• bzip2 + base64 encoded

Each hook point has various Key/Value variables injected into the script that can be used to template in the usual style.

Resolves #52