Mark markdown-css-paths safe as a file local var

[mattiasb]

Description

Make it possible to set markdown-css-paths as a file local variable.

This fixes the "file local variable" part of #834.

Related Issue

#834

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
  • I'm not sure if this makes sense to encode in a test.
    I'll look at it if you think it does though.
• All new and existing tests passed (using make test).

[mattiasb]

Thanks @syohex !