Introduce user option to translate absolute filenames

[phst]

Related Issue

#268

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 at the beginning of markdown-mode.el if needed.
• If documentation was added or changed, I have updated the README.md file using webpage.sh.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
• All new and existing tests passed (using make test).

[jrblevin]

This looks fine to me, but before committing I wondered if it should be generalized to allow translating all paths, not only absolute paths? For example, one use case might be to add a file extension to an extensionless URL. Any thoughts?

[phst]

Yes, that sounds simpler. Updated the PR.

[jrblevin]

Great, thanks for the patch!