Allow passing down a recent prop instead of localStorage

[Gargron]

Fix #129 - Since the only things the store is used for is the "recent emojis" functionality and skin modifier persistence, if you pass down recent emojis through props and showPreview=false, the store will never be hit.

[nightpool]

Since this adds a new property, it needs README documentation, right?

[nolanlawson]

LGTM, I appreciate that it just doesn't even write or read from localStorage at all if you pass in recent. It's conceivable users would want to handle this themselves or use some other store (IndexedDB, etc.).