feat: add --slide-visibility and --skip-hidden options for hidden slide handling

[eercanayar]

Summary

Adds two new flags for handling hidden slides in PowerPoint presentations:

• --slide-visibility - Inserts HTML comments (<!-- Status: hidden --> or <!-- Status: visible -->) for each slide
• --skip-hidden - Excludes hidden slides from the output while preserving original slide numbering

Motivation

This helps with:

• Identifying hidden slides in the converted markdown (e.g., draft slides, backup content)
• Filtering out hidden slides when you only want visible presentation content
• Maintaining slide number accuracy when skipping hidden slides (original numbering preserved)

Complements #90 (slide numbers) - together they provide complete slide metadata in the output.

Changes

• Added slide_visibility: bool = False and skip_hidden: bool = False to ConversionConfig
• Added --slide-visibility and --skip-hidden CLI arguments
• Added is_hidden: bool and original_index: int fields to GeneralSlide and MultiColumnSlide
• Added hidden slide detection via slide._element.get('show') == '0' in parser
• Added put_slide_visibility() method to Formatter base class
• Updated both Formatter.output() and QuartoFormatter.output() to use it

Example Output

With --slide-visibility:

<!-- Status: hidden -->
# Draft Slide

Content here...

---

<!-- Status: visible -->
# Published Slide

More content...

With --skip-hidden (hidden slides excluded, numbering preserved for use with --slide-numbers):

<!-- Slide 3 -->
# First Visible Slide

Content...

Testing

Tested with a 269-slide presentation containing hidden slides - correctly identifies and handles all hidden/visible states in both MD and QMD formats.