v0.6.0

• Add matchers .existsAtMostOnce() and .existsAtMostNTimes(x) #19
• Add selector .withParent(parent)/.withParents([...]) #21
• Add selector .withChild(child)/.withChildren([...]) #21
• Child selectors now only match children #22
• You can call act.tap() now with any WidgetSelector that returns a single widget #23