Add option to not embed images

[kfstorm]

Embedding images to markdown makes the file quite large. Markdown renderers such as Obsidian may be unresponsive during rendering.

• Option 1: Use external image links
  • Pro: File size is much smaller and reasonable.
  • Pro: Better performance in markdown renderers.
  • Con: Images may not be available in the future.
• Option 2: Download images as local files
  • Pro: Images are always available.
  • Con: Total file size is still large.
  • Con: Uploading multiple files to GitHub in one commit is much complicated.