holp it supports specifying observe container

https://github.com/ApoorvSaxena/lozad.js/blob/65f9744a47261960b8a3ba9b73b0f6050fc91192/src/lozad.js#L47-L49

if the `document` can be replaced by a specified dom, it will be greater!
Maybe like this:
```
observe( { container = document } ) {
const elements = container.querySelectorAll(selector)
}
```