add coverage track to playground example
add coverage track to playground example

snaheth/pileup.js,piotr-gawron/pileup.js,piotr-gawron/pileup.js,hammerlab/pileup.js,piotr-gawron/pileup.js,snaheth/pileup.js,hammerlab/pileup.js,snaheth/pileup.js,snaheth/pileup.js,piotr-gawron/pileup.js,hammerlab/pileup.js,hammerlab/pileup.js