Update index formatting logic

Update index formatting logic

* ivy.el (ivy--reset-state): Update.
(ivy--insert-prompt): Simplify.

If you want to see both the index and the length of the candidates,
instead of just the length, it can be done like this:

    (setq ivy-count-format "%d/%d ")

Each "%d" is replaced appropriately due to the total length of the
candidates. For instance, this one can result in "%4d/%-4d M-x ".

Re #167