Support textarea within a shadow DOM

[kojiishi]

I cannot use Wasavi in a textarea within a Shadow DOM. The site I edit daily use Shadow DOM, it's great if Wasavi can support that.

I found it's using activeElement, which needs slight update to work well with Shadow DOM. Please find this sample for how to get activeElement within Shadow DOM.

Wished to create a build environment to test locally and send PR but unfortunately I haven't got to there yet. It'd be great if you could have a look.