Feature request: Loader which waits to evaluate document.body until needed

[robacarp]

I'm wandering a bit from the readme with the install instructions and using the newer import style to load scroll-track. I believe that is causing some code to execute earlier than intended.

Adding only this to my project:

import scrollTrack from 'scroll-track';

Results in an error from getContentHeight

TypeError: document.body is null

Of course moments later document.body is available.