Add `counsel-kill-buffer'

[nbfalcon]

This way, a more convenient user-interface specifically to kill buffers can be
provided (ivy-rich, ...).

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[nbfalcon]

I'd probably need a copyright assignment here, since I already have 15 lines in ELPA.

[basil-conto]

I'd probably need a copyright assignment here, since I already have 15 lines in ELPA.

I thought the limit was per-package, but IANAL.

[basil-conto]

Either way, I'd recommend getting started on the paperwork, if you're willing.