added --new-file option for GNU diff compatibility

[deadPix3l]

Adds a new option to show file contents when one target does not exist.
If --new-file is passed and either a or b does not exist, it will diff the one that does exist against /dev/null

The effect of this is that the output will contain the full contents of the file, either every line showing as added or removed.
This provides helpful context beyond the default:

Only in /home/user/project/tests/.../: file1
Only in /home/user/project/tests/.../: file2
Only in /home/user/project/tests/.../: file3
...

You may ask "Okay the files are missing or added, thats maybe problematic? it all depends on what those files contain, however it doesnt show me that, so now I have to go and look at those files manually?" which is tedious and splits your outputs

NOTE: GNU diff also assigns the -N shorthand to --new-file but that cannot be the case here because it's already in use.

[jeffkaufman]

Thanks! Change makes sense and code looks good! Could you add a test?