Found this
https://itunes.apple.com/us/app/wifi...963277048?mt=8
but it's way more work than it ought to be. There really ought to be an app that simply reads network lists and lets you prioritize them. The app's name is a bit deceptive since what it really does it simply stop auto-joining for certain networks.
ETA: It also doesn't work. After installing per the instructions, my phone still auto-joined xfinity.