platform: add cygwin support

According to Brandon Jones, all we need to do is adjust /dev/clipboard
from xclip. So we add a platform specific file to do so.
http://www.relaytheurgency.com/2014/04/pass-in-cygwin-relatively-simple.html

Suggested-by: Brandon Jones <jones.brandon.lee@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>