Copy markdown-css-paths in the output buffer

[syohex]

Description

Related Issue

834 (CC: @mattiasb)

Type of Change

• [x] Improvement (non-breaking change which improves an existing feature)

Checklist

• [x] I have read the CONTRIBUTING.md document.
• [x] I have updated the documentation in the README.md file if necessary.
• [x] I have added an entry to CHANGES.md.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed (using make test).

[mattiasb]

That was fast! Hopefully I'll have time to take a check later today!

[mattiasb]

Tested and it seems to work fine!

[syohex]

Thanks for checking

[mattiasb]

Thanks for writing the code!