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

[silvester747]

The imports argument in the constructor of Template can be used to add arbitrary python code at the start of the generated template code. Due to the use of caching, we need to also set this in the TemplateLookup instances. As we only have a single instance of TemplateLookup for documents, start with always inserting the python path for all packages. If the need arises, we can always improve.