Fix extension issue in convert plugin when exporting a playlist

[pierreay]

Description

Fix extension substitution inside path of the exported playlist.

Before this, the exported playlist contained relative paths pointing to the
converted files BUT the extension were not substituted comparing to before and
the after the conversion. Therefore, running the playlist will fail for files
which have been converted and where extension have changed.

Example:

1. Convert /path/to/library/artist.flac to /path/to/converted/artist.mp3 using the -m playlist.m3u command-line flag.
2. Open the generated playlist, and find the incorrect path /path/to/converted/artist.flac inside.

To Do

• Documentation
• Changelog
• Tests