Make polyfill.io flags configurable

[snusson]

With setting the "flag=always" there is no further feature detection which seems to be great in first place.

import loadPolyfills from 'polyfill-io-loader!?IntersectionObserver

But this leads for the polyfills of e.g. IntersectionObserver to break in IE 11. With the flag always there is a polyfill made for getComputedStyle but this is not needed for IE11.

Please make the flag configurable from outside