add --node flag for filtering on pods from specific node

[skoef]

What this PR does / why we need it: While investigating a node with high CPU usage I figured I see what murre would show me. But knowing which node that had high CPU it wasn't super obvious which pods with high CPU usage were actually on that node. So I added a --node flag to murre allowing you to filter on a specific node, as well as you could already filter on namespace and pods.

While filtering on a node, murre still fetches metrics from all other nodes, but this is how murre was designed to operate. When you filter on a pod, all data for all other pods is collected as well, but not shown.

[skoef]

@maxlevinps Please take a look