Configuration file environment variable expansion

This commit adds simple variable expansion using either `$FOO`
or `${FOO}` when evaluating the config file. This can be disabled
by any command by using the `-no-expand-env` flag.

cmd/litestream/wal.go

@@ -17,7 +17,7 @@ type WALCommand struct{}
 // Run executes the command.
 func (c *WALCommand) Run(ctx context.Context, args []string) (err error) {
 	fs := flag.NewFlagSet("litestream-wal", flag.ContinueOnError)
-	configPath := registerConfigFlag(fs)
+	configPath, noExpandEnv := registerConfigFlag(fs)
 	replicaName := fs.String("replica", "", "replica name")
 	generation := fs.String("generation", "", "generation name")
 	fs.Usage = c.Usage
@@ -44,7 +44,7 @@ func (c *WALCommand) Run(ctx context.Context, args []string) (err error) {
 		}
 
 		// Load configuration.
-		config, err := ReadConfigFile(*configPath)
+		config, err := ReadConfigFile(*configPath, !*noExpandEnv)
 		if err != nil {
 			return err
 		}
@@ -118,6 +118,9 @@ Arguments:
 	    Specifies the configuration file.
 	    Defaults to %s
 
+	-no-expand-env
+	    Disables environment variable expansion in configuration file.
+
 	-replica NAME
 	    Optional, filter by a specific replica.