Add global config package.

This is a simple config system that I find myself wanting a lot.

config/config_test.go

@@ -0,0 +1,66 @@
+package config
+
+import (
+	"os"
+	"testing"
+)
+
+const (
+	testFilePath = "testdata/test.env"
+
+	// Keys
+	kOrder   = "ORDER"
+	kSpecies = "SPECIES"
+	kName    = "COMMON_NAME"
+
+	// Env
+	eOrder   = "corvus"
+	eSpecies = "corvus corax"
+	eName    = "northern raven"
+
+	// File
+	fOrder   = "stringiformes"
+	fSpecies = "strix aluco"
+	// Name isn't set in the file to test fall through.
+)
+
+func init() {
+	os.Setenv(kOrder, eOrder)
+	os.Setenv(kSpecies, eSpecies)
+	os.Setenv(kName, eName)
+}
+
+func TestLoadEnvOnly(t *testing.T) {
+	order := Get(kOrder)
+	species := Get(kSpecies)
+	if order != eOrder {
+		t.Errorf("want %s, have %s", eOrder, order)
+	}
+
+	if species != eSpecies {
+		t.Errorf("want %s, have %s", eSpecies, species)
+	}
+}
+
+func TestLoadFile(t *testing.T) {
+	err := LoadFile(testFilePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	order := Get(kOrder)
+	species := Get(kSpecies)
+	name := Get(kName)
+
+	if order != fOrder {
+		t.Errorf("want %s, have %s", fOrder, order)
+	}
+
+	if species != fSpecies {
+		t.Errorf("want %s, have %s", fSpecies, species)
+	}
+
+	if name != eName {
+		t.Errorf("want %s, have %s", eName, name)
+	}
+}