Adding skim support

[frzam]

See the Feature request here

Added skim support in kubens and kubectx go version.

Added environment variable PICKER which will contain value sk or fzf, if PICKER is not set or anything else is set other than sk and fzf, then it will by default pick fzf.

Created a func InteractiveSearch in interactive.go which will be called for fuzzy search and renamed fzf.go to fuzzy.go since it is now more generic.

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[ahmetb]

Hey, sorry I've been slow reviewing things!
For compatibility purposes, do you mind if we check fzf first, then sk? I am suspecting if people have another binary somehow called sk in their PATH, their workflow might be broken. (And this can work because I assume sk users don't have fzf installed?)

[ahmetb]

If this is not set then fzf is taken as default picker.

My expectation of this feature was that people can set this to anything and we can feature-proof the tool. e.g. imagine a tool named abc is developed in the future, I should be able to say PICKER=abc --opt1 -opt2.

I think this is how more common env vars like EDITOR or PAGER in posix systems work. The caller has no information about the program it's calling other than it knows how to pass it the information (either via xargs or via stdin).

[ahmetb]

Also this way, we can retain fzf logic and have an override via PICKER. The source code does not need to know about sk at all.

[sambonbonne]

For those who are waiting for this PR to be merged, you can create an alias with kubectx $(kubectx | sk), it's not a pefect solution but it works.

EDIT: typo