#230 Support overlapping fade effect

[Piotr Chlebek]

I got today, very popular error: sox FAIL fade: fade-out overlaps fade-in
when I was trying to cut 42ms fragment of audio with fade 25ms both sides.

You can consider to support above case to not fail.
The easiest way and the most intuitive way to implement it is just to allow overlapping fade effect - see attached image for triangular 25ms fade over 42ms fragment.
I don't see any reason why this path can't be choosen.

Love SoX,
Piotr