Support multiple servers/listeners. Breaking changes: - The top-level objects in www.yaml must be 'server'. - register_hndlrs() and register_hndlrs_methods() take a struct server_cfg parameter. - http_default_cfg() and http_cfg() set a required private field, so they must be called before starting a server and registering custom methods. Just populating a struct server_cfg is no longer sufficient. Add http_cfg(). Closes #3