Support vim-devicons, Nerd Font icons

As far as I know this plugin doesn't provide an API to interject icons to each filename and it's hard to make it work without changing and maintaining the internal code. [vim-devicons](https://github.com/ryanoasis/vim-devicons) provides this function to get icon out of extension of filename.

```vim
WebDevIconsGetFileTypeSymbol(filename, isdirectory(filename))
```

My suggestion is to add those icons automatically if `vim-devicons` plugin installed.

See https://github.com/ryanoasis/vim-devicons/issues/106 for details.

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