Added functionality to allow skipping Previews

[vinaygaba]

Fixes #158

One of the use cases of Showkase is to treat it as a catalog browser and only highlight specific composables in Showkase. Since Showkase automatically renders all @Preview composables as well, there was no way to skip those only from the Showkase browser. Composables that instead use @ShowkaseComposable can be skipped by merely removing the annotation from the function. However, if you remove the @Preview annotation, you lose out on the ability to see the preview in Android Studio as well. Hence, this PR introduces the ability to skip a composable from Showkase while retaining the ability to see the preview in Android Studio. All you need to do is to add the ShowkaseComposable annotation with the skip property. Here's an example

@ShowkaseComposable(skip = true)
@Preview(group = "Scrollable", name = "Vertical Scroll", showBackground = true)
@Composable
fun VerticalScrollPreview() {
    VerticalScrollSample()
}

@BenSchwab @elihart