Add migrate command

[majecty]

The new version of KeyStore has changed serialization format.
We should support users could migrate the old formatted file to a new formatted file easily.

[majecty]

There was an issue about this.
We need all passphrases in the old formatted file to migrate it.

I thinks that we could get each passphrase using interactive way.
Like this.

# A line starts with $ means shell command
# A line starts with > means cckey's output
# A line starts with < means user's input

$ cckey migrate old_formatted_file --keys-path new_file
> Start migration
> migrating 0x000000000001 address
> Please write your passphrase
< "my passphrase1"

> migratiing 0x000000000002 address
> Please write your passphrase
< "my passphrase2"

> migration complete