Add edge_label_box configuration to optionally remove edge label boxes

[Copilot]

Adds configuration to render edge labels without bounding boxes for cleaner diagrams.

Changes

• New configuration: edge_label_box (default: True) in BlockdiagExtension
• Drawing logic: Monkey-patches blockdiag's edge_label method to omit outline parameter when disabled
• Compatibility fix: Made md_globals optional in extendMarkdown for newer Markdown versions
• Documentation: Added configuration options section to README with usage examples
• Tests: Added test_edge_label_box_config to verify configuration behavior

Usage

# Python API
diagram = draw_blockdiag(content, edge_label_box=False)

# Markdown extension
markdown.markdown(text, extensions=['markdown_blockdiag'],
    extension_configs={'markdown_blockdiag': {'edge_label_box': False}})

# MkDocs
markdown_extensions:
  - markdown_blockdiag:
      edge_label_box: False

Implementation Notes

Only blockdiag uses outline parameter for edge label boxes. Seqdiag renders edge labels without boxes by default, so no patching needed. The monkey-patch approach avoids modifying upstream blockdiag library while providing clean API surface.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support Removal of Edge Label Box</issue_title> <issue_description>Optionally rendering Edge Labels without a bounding box could produce cleaner diagrams.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes gisce/markdown-blockdiag#13

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