Monotonic cubic interpolation extension

[csenszike]

Hi,

Thank you for your code, it's working perfectly.
In my project I have to interpolate monotonic key points and simple (natural) cubic interpolation sometimes gives non monotonic solution.
I've found the algorithm on Wikipedia with JS example code.
Would it be possible to improve the library with this option (:monotonic=>:boolean)?

Thanks in advance!

[tallakt]

Id be happy for a pull request, but right now I dont have much spare time to spend on this...