Example :

% tkn pipelineruns delete --all --keep=5

Will delete all pipelinesruns except the last 5 ones.

I think we should keep this option for the auto generated resources, like
taskruns and pipelineruns.

This was a query comping from a message on slack

https://tektoncd.slack.com/archives/CJ62C1555/p1581478036338900

asking if there was a way to limit the number of pipelinerun. There is no
support for this in pipeline atm but we can easily do that in cli and could be
much more flexible than what we can implement in pipeline.

/kind feature