Add folder support on file input

[ivmartel]

(As of 07/09/2017) Chrome >30, Firefox >50, Edge >14 and Opera >17 support the 'webkitdirectory' option for file input that allows to select / drag and drop folders. This must be implemented in the library,

Links:

• caniuse input-file-directory
• developer.mozilla.org webkitdirectory (Firefox 50 release notes)
• stackoverflow does-html5-allow-drag-drop-upload-of-folders-or-a-folder-tree