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

• [X] Documentation
• [X] Changelog
• [X] Tests

[pierreay]

I screwed up the commits. Sorry for the noise. I will perform two cleaned pull requests.