Add ability to customize currentRange setting in onSelectDay

[jhrabows]

While it is possible to do some customization of onSelectDay with current onSelect callback, the resulting range selection cannot be customized. One can think of use cases where a different implementation might be more appropriate. In many travel booking sites it is possible to modify one end of the selection range while keeping the other end unchanged. I recommend to keep the existing implementation as a default but allow it to be overwritten.