counsel.el: Make kill-ring filtering configurable

[basil-conto]

(counsel-string-non-blank-p): New function.
(counsel-yank-pop-filter): New defcustom defaulting to it.
(counsel--yank-pop-kills): Use it to destructively filter and
uniquify kill-ring and kill-ring-yank-pointer elements.
(counsel-yank-pop-action-remove, counsel-yank-pop):
Consistently test equivalence with equal-including-properties.

Re: #1356

[abo-abo]

Thanks.