Show visual indication of the selected namespace and context

[FabioAntunes]

When using kubens with fzf there's no visual indication of the selected namespace and context.
That's because fzf is considered a non-interactive tty, so there's no colour on the output.

This PR aims to fix that by creating a specific flag, that gets set every time we call kubens with fzf.

Fixes #98 and #89

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[FabioAntunes]

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[ahmetb]

I'm currently -1 for introducing yet another variable, some reasons:

• becomes yet another knob that takes effect only when fzf exists
• variable contains fzf (this was my bad, I have an existing variable like this) apparently other programs that do the same (e.g. fzy) exist too, so we need a generic name
• variable currently named KUBENS_ ideally it should be KUBECTX_

Simplest (but hacky) way I'm imagining we can solve this:

1. Force sending colors to fzf
2. fzf will show the colors
3. when user chooses an option, we'll get value with color sequences in it
4. strip off color sequences and use.

thoughts?

[ahmetb]

Please review #109. The if logic there is a bit different than yours, I wanna make sure we get it right.