Provide private network accounts and keys during creation.

To fetch keys from a private network requires making several calls to KMD. This is a chore for most testing that we can make easier.

Two options:

1) A new `--saveKeys ` option to create some machine readable file with the credentials.
2) A new `--printKeys` option that prints the information to stdout.

Some prior work has been done to extract the private keys from a private network:
https://github.com/winder/go-algorand/commit/3bff4a2bd2171cff321a48630c465ee84dfdfba5