Customization via Python hooks

[hjacobs]

Fixes #160:

• add --prerender-hook function to modify template context variables (e.g. to add arbitrary links)
• add --map-node-hook to allow changing the Node mapping (e.g. for custom pricing)
• add --map-pod-hook to allow changing the Pod mapping (e.g. for setting the application with custom logic)
• switch to argparse
• refactor output functions