Refactor ftintitle

Refactor the `ftintitle` plugin to move the extraction of the featured artist from the artist string into it's own function so that it can be tested.

This also fixes a bug that was in the original code that extracted the featured artist that would cause it to fail if the album artist was on the right side and the featured artist on the left side in the artist field. Also adds test cases to avoid this bug resurfacing.
Yes, please! Sounds perfect.