Don't optimize file if result is bigger than input

[Oondanomala]

When optimizing a gif, the result may be bigger than the input (there are already many issues about that)
The program could check if the output is bigger than the input and not optimize the gif if it is, just like a lot of other image compressors do.
Either an option to enable or disable this should probably be added too.

[kohler]

Feel free to submit a PR