Prefer props over library globals

[andryak]

The library defines a few globals including maxScale, minOpacity, scaleDuration and slideDuration.

It would be nice if the user could alter these values (e.g., setting maxScale to 1 to disable item scaling).

Please consider merging in MR #45 which removes such globals and replaces them with props with the same defaults. Since all these props are optional and their default values match the ones assigned to the globals we can guarantee backwards compatibility (thus the minor version bump).

[mochixuan]

I thought that users might not use these properties, so I wrote a fixed one and added it to props now. Thank you.