Control animation duration

Hello! Amazing work with the library, it's been great to use it so far.

I was wondering if you are planning to add support for passing duration as an option, since I only see [two types of animations](https://github.com/TahaSh/blendy/blob/f929b67cc5804b16c11346b5014da4c2ec064eb9/src/index.ts#L20) but no control over the actual animation duration. 

Thank you and keep up the good work!