Feature request: support an inverse operator on filtered search

[jmound]

Is your feature request related to a problem? Please describe.
The current filtering options allow for a subset of regular expressions. ie. On the pod list view for example, selecting filter (/) this works:

deafult|ns2|ns3|etc

However, there isn't full regex support. If, for example you wanted to view all pods not matching kube-system, this doesn't work:

!kube-system

Describe the solution you'd like
Full regex is cool, but if that's more complex (likely) than supporting just inverse searches, I think simple inverse searching would be a very helpful feature.

Describe alternatives you've considered
As suggested in the k9s slack, you can do k get po --field-selector metadata.namespace!=kube-system --all-namespaces

Additional context
The docs on filtering could use some more details- if you can use |, what else can you use?
Somewhat related: #564