Allow cluster-based theming along with context-based

[droopy4096]

this addresses some of the concerns from #2051 :+1:

added --theme-source option with two possible values: context or cluster in which case theme selection would be done either based on context name or cluster name.

For backward compatibility I've set default value for --theme-source to context

[droopy4096]

I have one issue: can't locate where is config.yaml being loaded - if I set k9s.themeSource: cluster - it still defaults to context, would appreciate input on where to look for that.