Allow multiple resources for sam build

### Describe your idea/feature/enhancement
I'd like to be able to pass multiple resources to `sam build`, e.g.: `sam build layer1 layer2 function1 function2`

The main reason I think this would be useful is I have a template that has multiple resources that I want to build individually. I end up having to build one, copy the output, build the other one, copy the output, etc, etc. It'd be nice if I could just specify multiple resources and have one template and set of resources as output.

I also have a large template that takes a while to run sam build against. I can currently specify one resource at a time, but then I can't take advantage of `--parallel` and it's kind of bulky to have to run the same command multiple times.

### Proposal

Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)
I don't think so?