Add a variable to fontify code block with a default mode

[sebastiencs]

In case there is no language specified on the code block

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.
• All new and existing tests passed (using make test).

[jrblevin]

Thanks! I changed the name to match markdown-edit-code-block-default-mode and converted it to a defcustom.