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/model.py

@@ -0,0 +1,26 @@
+from dataclasses import dataclass, field
+from typing import Optional
+
+from apischema import alias
+from yarl import URL
+
+from .fields import Host, Port, optional
+
+__all__ = ("MpdConfig", "Config")
+
+
+@dataclass(frozen=True)
+class MpdConfig:
+	password: Optional[str] = optional()
+	host: Host = Host("127.0.0.1")
+	port: Port = Port(6600)
+
+
+@dataclass(frozen=True)
+class Config:
+	schema: URL = field(
+		default=URL("https://cdn.00dani.me/m/schemata/mpd-now-playable/config-v1.json"),
+		metadata=alias("$schema"),
+	)
+	cache: Optional[URL] = optional()
+	mpd: MpdConfig = field(default_factory=MpdConfig)