cli: add the ability to output to /dev/stdout

[jzorn]

I have checked the following:

• I've searched existing issues and found nothing related to my issue.

Describe the feature you want to add

It is helpful to print the results of a collection run to stdout, as it is currently possible via bru run --output /dev/stdout. Some commands are adding additional output to stdout, e.g. when running a recursive collection Running Folder Recursively. This output prevents output processing in a pipeline, e.g. via bru run --output /dev/stdout | jq. By moving the non-result output to a different output stream, e.g. stderr, pipelines would be much easier to compose.

Mockups or Images of the feature

$ bru run --output /dev/stdout | jq
[ignored output on stderr]
{
  "summary": {
    "totalRequests": 1,
    "passedRequests": 1,
    "failedRequests": 0,
    "totalAssertions": 0,
    "passedAssertions": 0,
...
}

[jzorn]

Can we re-open this? @helloanoop