transition function implemented #2850

[coveralls]

Coverage remained the same at 95.197% when pulling fc3d6a0f8faf979bd6903a8167c56d25cdf319fc on mohammadali-seifkashani:master into c9b4b63fd08847281260205b995ae644f6f2f4d2 on jashkenas:master.

[jgonggrijp]

Hey @mohammadali-seifkashani, thanks for this very interesting contribution! There will be some more work to do and I might decide to put this in Underscore-contrib instead, but that's fine, we can make this ride together.

As a first step, could you add a few more commits to do the following? Thanks in advance.

• [ ] Rename the module to modules/transition.js (so without the leading _ in the name).
• [ ] Make the export default.
• [ ] List your function with the others in modules/index.js. I think it belongs in the utility section.
• [ ] Add tests for your function in test/utility.js.

I'll do a more detailed code review after this.