Add a dependency on `prom-client` and emit a basic latency metric for
testing purposes. Add a new configuration file for enabling/disabling
prometheus exporter and configuring the listen address.
Camo: https://github.com/atmos/camo. This has a couple advantages over
just allowing images to be dumped as-is:
- Prevents mixed-content warnings by allowing the server to proxy HTTP
images to an HTTPS camo instance
- Protects users' privacy by not exposing their browser directly to
the image host
- Allows the camo proxy to intercept and reject bad image sources
(URLs that are not actually images, gigapixel-sized images likely to
DoS users' browsers, etc.)
Whitelisting specific domains is supported for cases where the source is
known to be trustworthy.
