Allow specifying root selector/element for initial report

[sybers]

Hello,

First thank you for your work with this plugin :)

I have a use case a bit specific which is to use vue-axe on a styleguide powered by vue-styleguidist.

Because vue-axe takes the document as root node for axe-core to perform it's initial report, it generates a11y violations for things from vue-styleguidist. It'd be nice to be able to customise the selector for the initial check (which could default to document for backward compatibility) so it only analyses my components.

Thanks!