recommend an option for a `root`

[gillyspy]

current it is assumed that contentSelector will be applied to document. this is a fine assumption but i suggest that you be able to specify root as an option so that the badge can be added by those using an adhoc approach (e.g. a mutationObserver).

• in options set the root by default to document.
• suggest a check in loadto make sure root is eligible:

if(root !== document || !(root instanceof Element))
            throw new TypeError('root element is neither document nor an element')

• There are only a couple of lines where you would need to change document.querySelector to root.querySelector