Support reading multi-line ("pretty") JSON from a cmd

Need some option to enable a `cmd` to receive a single multi-line blob of JSON. Example use: AWS CLI, which seems to insist on emitting pretty JSON.

Example work-around:

```yaml
spec:
phases:
phase1:
steps:
- pub:
payload:
make:
name: awscli
type: cmd
config:
command: bash
doc: |
The AWS CLI insists on "pretty" JSON, which we
don't want. Here's a work-around using 'jq'.
args:
- '-c'
- 'aws iot describe-endpoint | jq -c'
- recv:
chan: mother
pattern:
success: true
- recv:
chan: awscli
pattern:
endpointAddress: "?addr"
```