Enable use of service role ARN when using change sets

[nullus]

I'm currently using stackup to build stacks across account. To do this I'm using both an assumed role (cross-account), and a service role which is passed to Cloudformation to perform the stack operations.

The standard up command supports the parameter --service-role-arn, but when using change sets this option isn't available.

I've created a PR to add this functionality.