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/fields.py

@@ -0,0 +1,28 @@
+from dataclasses import field
+from typing import NewType, Optional, TypeVar
+
+from apischema import schema
+from apischema.conversions import deserializer
+from apischema.metadata import none_as_undefined
+from yarl import URL
+
+__all__ = ("Host", "Port", "optional")
+
+T = TypeVar("T")
+
+Host = NewType("Host", str)
+schema(format="hostname")(Host)
+
+Port = NewType("Port", int)
+schema(min=1, max=65535)(Port)
+
+schema(format="uri")(URL)
+
+
+def optional() -> Optional[T]:
+	return field(default=None, metadata=none_as_undefined)
+
+
+@deserializer
+def from_yarl(url: str) -> URL:
+	return URL(url)