Issue - 54

Tabindex on focusable elements are not being restored to its original value after pinny close

Status: **Opened for visibility**

Reviewers: @scalvert @kpeatt 
Issue: https://github.com/mobify/pinny/issues/54

## Changes
- Instead of using $('[data-pinny-tabindex]') which is not supported in Zepto, we re-select focusable elements and filter out elements that does not have tabindex saved in data