Add getComposedRange to Selection

[rniwa]

Per WICG/webcomponents#79, we should add getComposedRange which takes a sequence of ShadowRoot to Selection and return a StaticRange.

[rniwa]

I think probably prioritize this over other issues since not being able to use selection is a major roadblock for shadow DOM users as far as I know.

[rniwa]

@smaug---- @annevk could you take a look at https://w3c.github.io/selection-api/#dom-selection-getcomposedrange and see if it makes sense?

[annevk]

Filed #161.