Support custom CSS styling and HTML output

[135yshr]

Background

Requested via Zenn comment: https://zenn.dev/link/comments/18e8c67d0561bd

カスタムcssでの修飾機能があるといいなと思いました（そのためにはHTML出力もあると便利？）

Proposal

Two related capabilities:

1. Custom CSS injection

Add a -css <path> flag (or -css <path>... to allow multiple) that injects user CSS after the built-in GitHub-flavored stylesheet so users can override or extend it.

md2pdf -css custom.css document.md

Use cases: corporate branding (logo colors, fonts), client-specific deliverable formatting, presentation-mode tweaks.

2. HTML output mode

Add a -html flag (or -format html) that stops the pipeline at step 3 (Build HTML) and emits the self-contained HTML instead of running Playwright.

md2pdf -html document.md   # produces document.html

Benefits:

• Lets users iterate on custom CSS quickly in a browser without spinning up Chromium each time.
• Useful for users who only want HTML (e.g. publishing to a static site).
• Removes the Playwright dependency for HTML-only workflows.

Design considerations

• Both flags should be composable: -html -css custom.css should produce HTML with the custom CSS applied.
• Custom CSS should be embedded inline in the <style> tag (not linked) to keep the HTML self-contained, matching md2pdf's current philosophy.
• Document precedence clearly: built-in CSS → custom CSS (latter wins).