Print all env vars prefixed with STELLAR.

[fnando]

What

$ stellar env
STELLAR_ACCOUNT=default          # use
STELLAR_CONFIG_HOME=/tmp/stellar # env
STELLAR_NETWORK=testnet          # use

$ stellar env STELLAR_CONFIG_HOME
/tmp/stellar

Why

So we can see all env vars, not just STELLAR_ACCOUNT and STELLAR_NETWORK.

Known limitations

N/A

[leighmcculloch]

Would be easier to maintain this list if it was in one place, but I assume it's not for a reason. So I suggest adding the below comments so when we edit it we know we need to modify the other place too.