Proposal: New default commands ExecPipeStdout() and ExecPipeStderr()

It would be nice if you can write a script like:

```
perform-api-request /someresource
[debug] exec-pipe-stdout jq .
cp stdout someresource.json
```

But right now you have to write something like:


```
perform-api-request /someresource
[debug] cp stdout tmp.json
[debug] exec jq . tmp.json
[debug] rm tmp.json
cp stdout someresource.json
```