[2.0] Configure should be generic

[DanielJette]

The new configure method should return a generic.

Configure always returns a ScreenshotTest<T> but this means that subclasses of the Rule (like ComposableScreenshotRule) must be invoked prior to configure. It breaks the logical flow of the test where it's common to want to configure first, then set your composables. The configure method should be something more like:

fun <TRule : ScreenshotRule<T>> configure(configureRule: TestifyConfiguration.() -> Unit): TRule {
    configureRule.invoke(configuration)
    return this as TRule
}