Add tuple support to template engine

[mitsuhiko]

This adds experimental tuple support to MiniJinja. It's a backwards incompatible change.

• Added proper tuple literal support to the template engine
• Tuples now render with Python-style formatting: (item1, item2, ...)
• Single-item tuples display with trailing comma: (item,)
• Tuples are functionally equivalent to lists but have distinct visual representation

Refs #785