Implement `querySelector` in Document

[cibernox]

Although the API of this DOM implementation intends to be minimal, would you deem acceptable to implement only querySelector and querySelectorAll on the document to enable some usages in ember-fastboot?

[RobbieTheWagner]

Any updates on this? This is essential for a lot of use cases.

[kevinansfield]

This would be very useful when simple-dom is used as part of mobiledoc-dom-renderer because it's often necessary to modify elements after the basic render.

[snewcomer]

Also looking at this to solve some interesting use cases.