Users should be able to tune the optipng optimization level from 0-7

While I'm sure there are multiple reasons you might want to use an optimization level other than the default of 7, I've found the greatest need for this when running integration tests. They can be painful slow when running heavy optimization with Piet.

This change makes it possible to specify a `level` option to the `optimize_png` call that you can use to adjust the default optimization level. The current default behavior (highest optimization level all the time) is preserved.