Add the metadata endpoint so that clients can get the server's
authorization and token endpoints.
Changes:
- Add a config object to allow administrators to configure the binding
address, binding port and domain name.
- Add a target in the magefiles and a step in the CI workflow to run
go vet.
- Add the Dockerfile.
- Replace the --address flag with the --config flag.
- Add the metadata endpoint to the router to return a JSON document
containing the URLs to the metadata and token endpoints.
- Set the project's placeholder name to indieauth-server.
- Update the magefiles and CI workflow.
- Add the version command to print the app's build information.
- Add the serve command to run the HTTP server.