Add command line parameter -o to specify the file name of the snapshot 

I'm trying to use deepin-scrot instead of scrot to capture screen in emacs, which require the screen capture program can specify the output file name from command line.
I modified deepin-scrot a little bit, adding a -o option to specify the output file name. If -o is used, it will save the snapshot to the file specified by -o when click finish button. If -o option is not used, the behaver will be the same (save to clipboard)
Please accept this pull request if you like the change and any comments are welcomed

Cheers!