2.0.0

This release is compatible with amphp/http-server@^3.

• A PSR-3 logger is now required in the Router constructor
• Starting the server with an empty Router is now allowed and only logs a notice instead of throwing an exception
• Middleware can be added via addMiddleware() instead of stack()
• Removed support for adding middleware directly via addRoute(), use stackMiddleware() instead if you need this
• Removed Router::onStart(), as it's now internally registered
• Removed support for ServerObserver, as it's been removed from amphp/http-server