adding the ability to maintain the placeholder color if desired

[mattaebersold]

@weotch I added this, which was to solve this issue on Clif https://app.asana.com/0/1199094171432804/1200386443419587/f

Problem:

• when the image is loaded, the check for showImage does 2 things
• 1. it fades in the image
• 2. it fades out the placeholder element
• Since both of those have a transition, you see the drop-shadow element for a second as the transition is happening on both
• My idea for the fix is to pass a prop called maintainPlaceholder which doesn't wrap .vv-placeholder in the transition

If you have an opinion on a better way to go about this, or at least how to clean up me needing to render the .vv-placeholder element twice, then let me know

This works locally on clif for me, and passing maintainPlaceholder as a prop lets me control this on a component-by-component basis, because I don't want this to happen on say, transparent pngs