ftintitle: New customization option to keep feature in artist field

[klb2]

Description

This PR adds a new option keep_in_artist to the ftintitle plugin.

Previous Behavior

The artist field is always changed to match the albumartist field.

New Behavior

The new option allows a customization of the old behavior. If the option is set to yes/true, the "feat. X" part is kept in the artist field. In order to be backwards compatible and not introduce unexpected behavior, the default is set to false, i.e., matching the old behavior.

Benefits

• Additional customization
• Searching for features using both title and artist (this can be very helpful when using music players with GUI)

To Do

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