Add capture interceptor

[DanielJette]

Is your feature request related to a problem? Please describe.

It would be cool if there was an easy way to intercept the captured bitmap, for logging, modification, or debugging.

There's already afterScreenshot on the ScreenshotLifecycle but this requires you to subclass the TestRule. It would be nicer if you could register any number of interceptors and have them called by Testify automatically, without having to subclass the rule.

This feedback relates to:

• [x] The Kotlin library
• [ ] The Gradle plugin
• [ ] The IntelliJ Platform plugin
• [ ] The sample code
• [ ] The documentation

Describe the solution you'd like

fun addBitmapInterceptor(interceptor: (Bitmap) -> Bitmap)

@ScreenshotTest
@Test
fun default() {
    rule
        .addBitmapInterceptor { bitmap ->
            Log.d("Testify", "The bitmap was captured")
            bitmap
        }
        .addBitmapInterceptor { bitmap ->
            val newBitmap = obfuscate(bitmap)
            newBitmap
        }
        .assertSame()
}