support custom Kubernetes cluster name shortening with regex

[joemiller]

Adds two new flags:

• • -shorten-kube-names-regex-match
• • -shorten-kube-names-regex-template (defaults to ${1})

Examples:

Assume a kubernetes cluster named myorg-region-profile-cluster-01

Using the default template:

powerline-go -shorten-kube-names-regex-match '^myorg-\w+-\w+-(.*)'

result: cluster-01

Using a custom template:

powerline-go -shorten-kube-names-regex-match '^myorg-(\w+)-\w+-(.*)' -shorten-kube-names-regex-template '${1}/${2}'

result: region/cluster-01