Add a worker for pngquant.

[adammathys]

Simple implementation of the worker for pngquant. I noticed there was interest in issue #14 so I figured I'd submit a pull-request in case you'd like to use this as a starting point.

[toy]

Thanks for a good starting point.
I'll try to have a look soon.

[LeonB]

Any progress on this point? Is there something we can do?

[adammathys]

Sorry, I had all but forgotten about this pull-request. I've rebased it against master, is there anything you'd like me to change or add @toy?

[kaspergrubbe]

Does it make sense to take Pngquants --quality parameter into account? And what does it default to?

[toy]

@adammathys @LeonB @kaspergrubbe Sorry for such a long wait, I am working on this now.
I'll check which parts of code to use from this pull request and from #40. I will change two important things:

1. There is an --output argument in pngquant starting from version 2.
2. --quality must be used as default value will give lossy conversion instead of skipping worker for images with more than 256 colours. Quality for pngquant can be specified in multiple forms (M, -M, N- and N-M), but I am thinking of an option allowing only explicit range or two options for min and max values.

[toy]

@adammathys @LeonB @kaspergrubbe Please have a look at #52