Support a TOML configuration file

The new config file currently only configures the same options that were
already available through environment variables. However I have ideas
for additional features that would be much nicer to support using a
structured configuration format like TOML rather than environment
variables, so config files exist now!

The previous environment variables are still supported and will be used
if you don't have a config file. I plan to keep supporting the MPD_HOST
and MPD_PORT environment variables forever since they're shared with
other MPD clients such as mpc, but I may eventually drop the environment
variables specific to mpd-now-playable in a future release.

src/mpd_now_playable/config/schema.py

@@ -0,0 +1,28 @@
+from json import dump
+from pathlib import Path
+from pprint import pp
+from typing import Any, Mapping
+
+from apischema.json_schema import JsonSchemaVersion, deserialization_schema
+
+from .model import Config
+
+
+def generate() -> Mapping[str, Any]:
+	return deserialization_schema(Config, version=JsonSchemaVersion.DRAFT_7)
+
+
+def write() -> None:
+	schema = dict(generate())
+	schema["$id"] = Config.schema.human_repr()
+
+	schema_file = Path(__file__).parent / Config.schema.name
+	print(f"Writing this schema to {schema_file}")
+	pp(schema)
+	with open(schema_file, "w") as fp:
+		dump(schema, fp, indent="\t", sort_keys=True)
+		fp.write("\n")
+
+
+if __name__ == "__main__":
+	write()