Save Blockchain

[NewtonJr4108]

Blockchain doesn't save when miner mines blocks. This is a key feature that I think this project deserves.

[Ali-Xoerex]

@cosme12
I believe if we implement an interface for interacting with a database , both issues #90 and #7 will be fixed and closed.
My suggestion is something simple like SQLite. If you think it's reasonable too , please assign me to this issue. I'd like to work on it