iffy does not support extended function semantics

[eprykhodko]

Any reason why it doesn't?
I think it should, at least in the form with 2 or 3 arguments.

In [28]: from funcy import iffy
In [29]: foo = iffy({'x',  'y'}, len)
In [30]: foo('x')

The above feels like it should be a valid use of funcy.
Thanks!

[Suor]

Implemented by @eprykhodko.