Add support for local backup

[0x3639]

Add the ability to backup the blockchain locally with a --backup flag.

Can we specify height? In the past I've simply stopped go-zenon and copied the necessary files. Backup by height is a different story.

[0x3639]

For this enhancement I'm thinking we:
stop go-zenon
create a directory called ~/znn.bak/{datetime}
tar .znn and place it into the necessary folder
start go-zenon