add full TAB highlight when switching between buffers 

继上次[讨论](https://github.com/techlivezheng/vim-plugin-minibufexpl/issues/20)
这个PR解决了这个issue [color highlight current selected](https://github.com/fholgado/minibufexpl.vim/issues/23)
[效果预览](http://screencast.com/t/ZkaRUtL1f)