Allow command for optimizing single file with --output name specified

[mrdeveloperdude]

This kind of tool usually has an optional command line available like this:

tool --output my_output_image.png my_input_image.png

This is very useful, especially in cases where you want to keep the original un-optimized file.

Another form would be

tool source_dir --output-dir destination_dir

where it is understood that all files found recursively under source_dir will be converted to the same dir under destination_dir.

[victordomingos]

Yes, I agree. Thanks for the suggestion.

At this time I am a bit occupied and can’t take this task, but I am keeping it as a reminder. I may accept a pull request if someone decides to go ahead with it.