Add support for client hint headers

Client Hints are defined by draft-grigorik-http-client-hints and are supported in the recently-released-to-stable Chrome 46. They allow user agents to provide additional hints regarding how the content is going to be rendered. Particularly interesting for imageproxy:

• DPR (Device Pixel Ratio) - could be used to override the requested size options to accomodate higher DPR values
• Width - override requested width, since the client hint indicates the width at which the image is actually going to be displayed
• Downlink - could be used to change the default quality setting for JPEG images (?)

We may want to hide this behind a flag, at least for the time being, just to make sure it doesn't cause any problems. We'll also need to make sure and set an appropriate Vary header.

Further Reading:

• https://developers.google.com/web/updates/2015/09/automating-resource-selection-with-client-hints
• https://github.com/igrigorik/http-client-hints

No source file, test, or entry point is named. Start by reading the Client Hints draft and the imageproxy request and response handling, then determine which hints and flag behavior are in scope; done should include appropriate Vary handling and tests for the selected behavior.