Add ERC20MintableBurnable

[yudetamago]

Add mint, burn, and burnFrom functions to ERC20.

refs (zeppelin's implementation):

• https://github.com/OpenZeppelin/openzeppelin-solidity/blob/6407d7818d3d101ca834b126dbd8f902e5b06c8a/contracts/token/ERC20/ERC20.sol
• https://github.com/OpenZeppelin/openzeppelin-solidity/blob/6407d7818d3d101ca834b126dbd8f902e5b06c8a/contracts/token/ERC20/ERC20Burnable.sol
• https://github.com/OpenZeppelin/openzeppelin-solidity/blob/6407d7818d3d101ca834b126dbd8f902e5b06c8a/contracts/token/ERC20/ERC20Mintable.sol

[nrryuya]

ERC20MintAndBurnable ? 🤔