Creating an `android.graphics.drawable.Icon` from Iconics seems to be clunky
There doesn't seem to be any way to set a notification icon (large or small) directly from a `Drawable`.

As far as I can tell, it's possible to create an `android.graphics.drawable.Icon` from Iconics in the following way:
>```Icon.createWithBitmap(IconicsDrawable(context).icon(GoogleMaterial.Icon.gmd_directions_bike).sizeDp(24).toBitmap())```

Compared to `Icon.createWithResource(context, R.drawable.directions_bike)`, this is pretty clunky.  I'd like to think it would be possible to (at worst)! do something like:
>```IconicsDrawable(context).icon(GoogleMaterial.Icon.gmd_directions_bike).toDrawableIcon()```

without having to specify the size.

Would you agree?  If so, would you consider a PR to add the suggested implementation, or would you want to do it in a different way?