Add a "custom-placeholder" default style for showLoadingAnimation

[burkeshartsis]

Currently if you pass showLoadingAnimation while using a custom placeholder you have to make sure your custom placeholder includes one of the the default style class decelerations. Adding a custom-placeholder class here and passing it by default to all custom placeholders would mean <ReactPlaceholder customPlaceholder={<StyledLine />} showLoadingAnimation /> would just work ⚡