Consider allowing light DOM and shadow DOM wrappers separately

[treshugart]

Currently there's only mount(). There might be use cases for:

• light() - Only traverses light DOM
• shadow() - Current impl of mount(). Uses shadowRoot, but does not descend into descendant components contained in the shadow root (only sees their light DOM).
• shadow({ deep: true }) traverses child shadow roots all the way down

[treshugart]

This might cover #18 re shadow({ deep: true }).