Feature: Autoload submodules.

- [ ] Check if something like this already exists
- [ ] Figure out how to handle differing options.
- [ ] Consider how outputs of submodules work.
- [ ] Update doc examples to show both.
- [ ] Experiment if a map within a list can work or like a map of nested maps.

Idea:

```hcl
module "serverless" {
source = "FormidableLabs/serverless/aws"

region = "us-east-1"
service_name = "sparklepants"
stage = "${var.stage}"

modules = [
"xray" # submodules/xray/
]

plugins = [
"serverless-s3-sync" # submodules/plugin-serverless-s3-sync/
]
}
```