Add read/write support for custom document properties.

• document.custom_properties — CustomProperties collection
• custom_properties.add(name, value) — supports string, int, float, bool, datetime
• custom_properties[name] — get by name
• Iterable: for prop in custom_properties

These are stored in docProps/custom.xml, separate from core properties.

Files: docx/custom_properties.py (new), docx/oxml/custom_properties.py (new), docx/parts/custom_properties.py (new)