Add label filtering to cluster-list

[DazWilkin]

I was unable to find a way (without using jq) to filter clusters:

linode-cli lke clusters-list --label=${LABEL} ...

In a similar vein to:

linode-cli linodes list --label=${LABEL} ...

This is a useful way to grab one (few) of many results.

Am I overlooking equivalent functionality in the CLI for LKE?