Configurable python path per package

[silvester747]

Support configuring a path containing python scripts to be included in the documentation. The path will be added to the python path in the documents before rendering.

Allow specifying a python path per package and on the command-line

• [x] Extend package definition with python path
• [x] Extend command line options with default python path
• [x] Extend renderer to include python paths in templates
• [x] Extend documentation