Fix OptiPNG not being able to overwrite existing file
Fix OptiPNG not being able to overwrite existing file

Fixes #20

OptiPNG issue: https://sourceforge.net/tracker/?func=detail&aid=3607244&group_id=151404&atid=780913

vladikoff/grunt-contrib-imagemin,bevacqua/grunt-contrib-imagemin,lstaszak/grunt-contrib-imagemin,gruntjs/grunt-contrib-imagemin,guardian/grunt-contrib-imagemin