Crossfade/mix, Equalizer

[migzserrano18-del]

add crossfader, gapless songs, options. and add Equalizer for those who wants to tinker with it. possibly, add custom mixing songs

[loreofmin-ux]

Thanks for the ideas! The equalizer is also requested in #37, so it's worth adding your vote there. Crossfade and gapless playback aren't in the works yet, as far as I know, and a PR would be welcome.