Add ZopfliPNG option to keep timestamp of image file about zopfli HOT 3 OPEN

Workaround (for Windows):
After ZopfliPNG and before deleting the original file, call the attached batch 
(it calls the attached PowerShell script that clones lastWriteTime and 
CreationTime).

The option to keep the last modified time is already there:

--keepchunks=tIME

Storing the creation time is trickier, as this is stored in a text node that 
may have other values: 
http://www.libpng.org/pub/png/book/chapter11.html#png.ch11.div.2

I meant the file timestamps (creation, last modification) in the file system, 
not the timestamp in the tIME chunk.