Use first subdomain of host argument to fetch IAM authentication details

[robins]

Use first subdomain of host argument to fetch IAM authentication details

So when psql is run with this:
psql -U abc -h redshift2.abcd.ap-southeast-2.redshift.amazonaws.com

We use redshift2 as the --cluster-identifier for the IAM CLI call

[robins]

There is a corner-case for this task that is marked in ( Issue #36 ) but this one's done.