Initial index keyword argument

I added :initial-index keyword argument to `popup-menu*' function.

I want to use this feature for my IME program `sekka.el'.
Please merge this...

![demo_of_initial_index](https://cloud.githubusercontent.com/assets/80378/3764992/81d9c4c0-18b7-11e4-8374-25e4f5f0c4fd.png)

I agree with that.
Your code is simple and clean.