Fill fields inside shadow roots

[raxod502]

Fairly straightforward. I just introduce a function queryShadowRoots that converts a single parent element into a list that contains the parent plus all shadow roots amongst its descendants. Then queryAllVisible invokes it and iterates through the results, applying querySelectorAll to each individual shadow root (and the original parent).

I tested this and it seemed to work right away, allowing autofill to succeed on one of my sites where the login form is inside a shadow root.

I recommend viewing the diff with whitespace changes disabled.

Closes #73