Force PNG output

The tool is more composable and unix-y if the output is always png, for (at least) two reasons:

1. The output is predictable, and the tool can appear in pipelines that can not automatically tell a jpeg from a png

2. png is lossless. This means that the tool does one thing: it renders a snapshot of the page. If the user wants lossy compression, she can easily add that later.

Also, jpeg compression of web sites is prone to creating artefacts along sharp edges. I don't think it is a good default.