support erc721-c which provides more flexible royalty policy and is supported by opensea.

[foxgem]

FYI

reference: https://medium.com/limit-break/introducing-erc721-c-a-new-standard-for-enforceable-on-chain-programmable-royalties-defaa127410

[foxgem]

@oleggrib looks good to me. I also assigned this to @JustinFeng for a second review, thanks.