v2.0.0

This release is largely a refactoring release to remove the defunct concept of "levels" from jsdom, in favor of the living standard model that browsers follow. Although the code is still organized that way, that's now noted as a historical artifact. The public API changes while doing so were fairly minimal, but this sets the stage for a cleaner jsdom code structure going forward.

• Removed: jsdom.level, and the level option from jsdom.jsdom.
• Change: the nonstandard Element.prototype.matchesSelector method was replaced with the standard Element.prototype.matches. (@kenpowers)
• Fix: querySelector correctly coerces its argument to a string (1.2.2 previously fixed this for querySelectorAll).