fix: only load polyfill when dom is loaded

[NickDJM]

The polyfill tries to load itself immediately, which can cause errors if the DOM isn't loaded yet.

This PR implements a check to see if the DOM is loaded and will not run the support check until it has.

Fixes #7

[NickDJM]

Tested on Chrome, Firefox, Safari, (old) Edge, and IE.

All work as expected.

[rstacruz]

Thanks for this, I'll check it out when I get the time!