Add global config package.

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

config/config.go

@@ -0,0 +1,84 @@
+// Package config implements a simple global configuration system that
+// supports a file with key=value pairs and environment variables. Note
+// that the config system is global.
+package config
+
+import (
+	"bufio"
+	"log"
+	"os"
+	"strings"
+)
+
+// NB: Rather than define a singleton type, everything is defined at
+// the top-level
+
+var (
+	vars   = map[string]string{}
+	prefix = ""
+)
+
+// SetEnvPrefix sets the prefix for all environment variables; it's
+// assumed to not be needed for files.
+func SetEnvPrefix(pfx string) {
+	prefix = pfx
+}
+
+func addLine(line string) {
+	if strings.HasPrefix(line, "#") || line == "" {
+		return
+	}
+
+	lineParts := strings.SplitN(line, "=", 2)
+	if len(lineParts) != 2 {
+		log.Print("skipping line: ", line)
+		return // silently ignore empty keys
+	}
+
+	lineParts[0] = strings.TrimSpace(lineParts[0])
+	lineParts[1] = strings.TrimSpace(lineParts[1])
+	vars[lineParts[0]] = lineParts[1]
+}
+
+// LoadFile scans the file at path for key=value pairs and adds them
+// to the configuration.
+func LoadFile(path string) error {
+	file, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		addLine(line)
+	}
+
+	if err = scanner.Err(); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// Get retrieves a value from either a configuration file or the
+// environment. Note that values from a file will override environment
+// variables.
+func Get(key string) string {
+	if v, ok := vars[key]; ok {
+		return v
+	}
+	return os.Getenv(prefix + key)
+}
+
+// GetDefault retrieves a value from either a configuration file or
+// the environment. Note that value from a file will override
+// environment variables. If a value isn't found (e.g. Get returns an
+// empty string), the default value will be used.
+func GetDefault(key, def string) string {
+	if v := Get(key); v != "" {
+		return v
+	}
+	return def
+}