Add changelog

To support the keepachangelog.com cause..

Signed-off-by: Byron Ruth <b@devel.io>
Add support for configuration file

A JSON-based configuration file can be provided at startup time for the
service, either with a command line flag `-c` or an environment variable
`ORIGINS_CONFIG`.

Any other environment variables take precedence which enables
environment-specific overrides to a base configuration.

Signed-off-by: Byron Ruth <b@devel.io>