sometimes previously installed or staging icons can't be deleted anymore, even not from admin..

hi,
sometimes npm has difficulties to delete previously installed icons:

```
npm verb node v8.9.3
npm verb npm v5.5.1
npm ERR! path C:\xxx\node_modules\.staging\material-design-icons-fdc6f299\av\drawable-xxxhdpi\ic_mic_none_black_48dp.png
npm ERR! code EPERM
npm ERR! errno -4048
npm ERR! syscall unlink
npm ERR! Error: EPERM: operation not permitted, unlink 'C:\xxx\node_modules\.staging\material-design-icons-fdc6f299\av\drawable-xxxhdpi\ic_mic_none_black_48dp.png'
npm ERR! { Error: EPERM: operation not permitted, unlink 'C:\xxx\node_modules\.staging\material-design-icons-fdc6f299\av\drawable-xxxhdpi\ic_mic_none_black_48dp.png'
npm ERR! stack: 'Error: EPERM: operation not permitted, unlink \'C:\\xxx\\node_modules\\.staging\\material-design-icons-fdc6f299\\av\\drawable-xxxhdpi\\ic_mic_none_black_48dp.png\'',
npm ERR! errno: -4048,
npm ERR! code: 'EPERM',
npm ERR! syscall: 'unlink',
npm ERR! path: 'C:\\xxx\\node_modules\\.staging\\material-design-icons-fdc6f299\\av\\drawable-xxxhdpi\\ic_mic_none_black_48dp.png' }
npm ERR!
npm ERR! Please try running this command again as root/Administrator.
npm verb exit [ -4048, true ]
```

the problem is, even I am not able to delete it by hand and also delete it with admin rights is not possible.
this problem occurs only in your lib, why? I have to logoff and login again to able to delete it..
**OS: Windows 10**

Start with the reported node_modules/.staging/material-design-icons-fdc6f299/... path and reproduce the deletion failure on Windows 10 using Node v8.9.3 and npm 5.5.1 from the log. Check whether the icon library leaves the PNG unavailable until logoff; done means identifying a reproducible cause and verifying a fix or workaround.

Written by the indexing model from the issue text.