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/load.py

@@ -0,0 +1,34 @@
+from os import environ
+
+from apischema import deserialize
+from pytomlpp import load
+from xdg_base_dirs import xdg_config_home
+
+from .model import Config
+
+__all__ = "loadConfig"
+
+
+def loadConfigFromFile() -> Config:
+	path = xdg_config_home() / "mpd-now-playable" / "config.toml"
+	data = load(path)
+	print(f"Loaded your configuration from {path}")
+	return deserialize(Config, data)
+
+
+def loadConfigFromEnv() -> Config:
+	port = int(environ["MPD_PORT"]) if "MPD_PORT" in environ else None
+	host = environ.get("MPD_HOST")
+	password = environ.get("MPD_PASSWORD")
+	cache = environ.get("MPD_NOW_PLAYABLE_CACHE")
+	if password is None and host is not None and "@" in host:
+		password, host = host.split("@", maxsplit=1)
+	data = {"cache": cache, "mpd": {"port": port, "host": host, "password": password}}
+	return deserialize(Config, data)
+
+
+def loadConfig() -> Config:
+	try:
+		return loadConfigFromFile()
+	except FileNotFoundError:
+		return loadConfigFromEnv()