Picture tags are not working in IE11.

Steps to Reproduce

If you go to demo site https://apoorv.pro/lozad.js/demo/ and scroll to the picture tag example you will see the next error by IE console:
SCRIPT438: Object doesn't support property or method 'append'
File: lozad.min.js, Line: 9, Column: 323

Possible Solution

I solved with the this pollyfill, but there may be better solutions
https://developer.mozilla.org/en-US/docs/Web/API/ParentNode/append#Polyfill

If you could include in the documentation the best solution for this and solve the problem in the example page it would be great, thank you!