Add kubernetes example

[jsantiagoh]

First, I have to say that I love this tool, I've been using it a lot for this particular case and it works amazingly well.
This example can be used by running rat --mode contexts --cmd='kubectl config get-contexts -o=name'

I have more changes on my fork that I'm happy to merge individually or all of them if you are interested.

• Split by using the golden ratio, so the active panel is larger than their parents
• Use go modules

[ericfreese]

Awesome! Glad you've found it helpful. Thanks for sharing your config. Would definitely be interested in other changes. Feel free to open more PRs.