Add `customTrackBy` prop

[thecodewarrior]

This is pretty simple, it just adds a customTrackBy prop, which works similarly to customLabel. This should solve #701

Note: You'll have to run npm run bundle before merging this, I wasn't able to run it myself. For some reason npm install froze every time I tried to run it (probably an issue with running an M1 mac), so I had to install the dependencies with yarn. That worked, but I couldn't use the lockfile, so dependencies might be different versions. I don't want to break anything, so I opted not to do the final build myself.