pretty print parameters for new stack

Now looks like:

```sh
$ bin/cfn-clone -s source-stack -n new-stack -a Role=FOOBAR
The merged parameters are:
Abort                           no
AmiId                           ami-1234
AvailabilityZones               us-west-2a,us-west-2b
InstanceType                    m3.large
MaximumInstances                1
MinimumInstances                1
Role                            FOOBAR

Going to clone
...
```

Fixes #3