Prop types, runtime type checking, and component code

[mixxorz]

This PR:

• Adds a way to define prop types
• Adds a way to define default values for props
• Adds optional runtime type checking
• Adds a way to add custom Python code to components

Example component:

---
props.types = {
    'required_string': str,
    'optional_number': Optional[int],
    'default_number': int,
}
props.defaults = {
    'default_number': 10,
}

# Arbitrary Python code that adds variables to the template context
props['new_number'] = props['default_number'] * 2

# props['default_number'] will return 10 if `default_number` is not passed to the component.
---

The context contains:

Required string: {{ required_string }}
Optional number: {{ optional_number }}
Default number: {{ default_number }}
New number: {{ new_number }}