add parameter to limit number of iterations

[nekromoff]

Since guetzli is very slow, it would be helpful if user could limit number of iterations compression goes through. This would speed things up even if not reaching maximum compression possible.

[tomByrer]

Have you tried adjusting the quality levels? "if not reaching maximum compression possible", why then use Guetzli? There are other options out there that can get to 90-95% of Guetzli's file sizes in seconds.

[nekromoff]

I still think this would help the speed. Just check the issues, multiple users complaining about the slowness.