Add filepath flag to cfn-config

This allows the user to specify a local or S3 path when running cfn-config. For example:

```sh
# Write to a local path
cfn-config -r us-east-1 -n my-stack -t /path/to/my/template -f ~/configuration/my-stack.json

# Write directly to S3
cfn-config -r us-east-1 -n my-stack -t /path/to/my/template -f s3://my-bucket/my-folder/my-stack.json

# Filepath is optional, this writes to `cwd`/my-stack.cfn.json like it did before
cfn-config -r us-east-1 -n my-stack -t /path/to/my/template
```

cc @willwhite @yhahn 

p.s. if we decide to switch to positional args, lets just do that all at once. Also, filepath is optional but I still managed to remove the reference to StackName from #34.