Add feature of refreshing converted files if original files are modified

[pierreay]

Description

By default, the convert plugin will skip every pending conversion if the converted file is already existing on the filesystem. However, when making changes (e.g., tags) in the original files, the changes are not reflected to the converted library. Adding the --refresh command-line option and the refresh option to the convert plugin allows to re-convert the original files if they are modified more recently than the time when converted files were converted.

To Do

• Documentation.
• Changelog.
• Tests.