New Custom pipes

[KostyaDanovsky]

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I think it will be great if this library can have additional pipes like isEmpty, any.

In your example $items | length | not | async it's kind a hard to understand what do you mean there, it will be much easier to understand $items | isEmpty | async

[neversaid]

I have added a new custom pipe (isEmpty) for that and created a PR #3 . It checks Arrays and Strings for "empty".