allow skin to be selected via K9S_SKIN env var

[trever-atr]

minimal code change to allow the skin to be overriden by an environment variable K9S_SKIN.

config.ui.skin overwrites the default, context.skin overwrites the config setting, and K9S_SKIN overwrites the context setting. all of these are optional.

among others, this enables a use case where the active shell session can influence the k9s skin to distinguish between different clusters, without having to manually manage many per-cluster, per-context config files.

this solves https://github.com/derailed/k9s/issues/1909 and a couple related issues.

i have verified functionality and it works as desired.

@derailed please let me know if there are any changes required to get this merged, thanks!

[trever-atr]

i have added K9S_SKIN to the README ;)

[trever-atr]

@derailed can we get this merged now? :)