[Feature request] Integrate PNGQuant Dithering + Posterize Options

[philippedcote]

Quick, short summary:

• Dithering and Posterization cannot be controlled in the current implementation of PNGQuant.

Expected results:

• Let the user control Dithering (--floyd=N) on a scale from 0 to 1 (UI could be a Slider with 0.1 increments).
• Let the user Enable + define Posterization (--posterize bits N) from 2 to 256 colors (UI could be a CheckBox + Text Field).

Actual results:

• Current PNGQuant implementation only let the user control quality / speed.

A link to download a simplified project or file that shows the issue:

• Here is a Project file with few examples of results. 2017-04-27-CodeKitPNGQuant.zip