Introduce native NFT support

[pauliusuza]

I wrote a proposal for adding native Non Fungible Token (NFT) functionality in steem.

The proposal allows for many interesting use cases allowing third parties to modify the NFT data and goes beyond simple non-fungible tokens. Hope to start a discussion that would lead to this feature landing in steem codebase.

https://github.com/thesweatshop/tokenforge/blob/master/SIP/SIP001.md