added default suffix for world save to PNG
added default suffix for world save to PNG

fix #34: now a default suffix ".png" is added when a filename without a
suffix was given.

EtlamGit/minutor,mrkite/minutor,Metibor/minutor,Metibor/minutor,mrkite/minutor,EtlamGit/minutor,mrkite/minutor,EtlamGit/minutor,Metibor/minutor,Metibor/minutor,mrkite/minutor,EtlamGit/minutor,EtlamGit/minutor,Metibor/minutor,mrkite/minutor