- bump package version to 0.78.0
    - cash `ul.it-select-list` element in `dropdownRef`
        | to use it in `scrollToSelectedOption`
    - make `selectOption` use option's index instead of `option: TOption`
        | to set `selectedOptionIndex` to get with it
        | the current selected option's reference
    - make `scrollToSelectedOption` set `scrollTop` of `scrollerRef`
      with `offsetTop` of `selectedOption`

close Equal-UI#250