Add startTime and endTime

[lassoan]

Thanks a lot for this convenient package!

Could you add startTime and endTime options so that we can easily assemble more complex animations just by adding multiple functions? If time<startTime then the function would return start value; if time>endTime then function would return end value.

Example:

expandScaleFactors = easing_functions.CircularEaseInOut(start=1, end=2, startTime=0.0, endTime=2.0)
contractScaleFactors = easing_functions.CircularEaseInOut(start=0, end=-1, startTime=4.0, endTime=6.0)
...
scaleFactor = expandScaleFactors(t) + contractScaleFactors(t)