Walk parent directories to find config file (#141)

Code is mostly borrowed from [gqlgen](https://github.com/99designs/gqlgen).

The idea here is that I want to be able to store `genqlient.yaml` at the top-level, but my client code lives down in `graph/client/`. I put the `//go:generate` line in `graph/client/client.go`.

generate/main.go

@@ -14,9 +14,18 @@ import (
 )
 
 func readConfigGenerateAndWrite(configFilename string) error {
-	config, err := ReadAndValidateConfig(configFilename)
-	if err != nil {
-		return err
+	var config *Config
+	var err error
+	if configFilename != "" {
+		config, err = ReadAndValidateConfig(configFilename)
+		if err != nil {
+			return err
+		}
+	} else {
+		config, err = ReadAndValidateConfigFromDefaultLocations()
+		if err != nil {
+			return err
+		}
 	}
 
 	generated, err := Generate(config)
@@ -42,7 +51,7 @@ func readConfigGenerateAndWrite(configFilename string) error {
 }
 
 type cliArgs struct {
-	ConfigFilename string `arg:"positional" placeholder:"CONFIG" default:"genqlient.yaml" help:"path to genqlient configuration (default genqlient.yaml)"`
+	ConfigFilename string `arg:"positional" placeholder:"CONFIG" default:"" help:"path to genqlient configuration (default: genqlient.yaml in current or any parent directory)"`
 	Init           bool   `arg:"--init" help:"write out and use a default config file"`
 }
 
@@ -67,7 +76,12 @@ func Main() {
 	var args cliArgs
 	arg.MustParse(&args)
 	if args.Init {
-		err := initConfig(args.ConfigFilename)
+		filename := args.ConfigFilename
+		if filename == "" {
+			filename = "genqlient.yaml"
+		}
+
+		err := initConfig(filename)
 		exitIfError(err)
 	}
 	err := readConfigGenerateAndWrite(args.ConfigFilename)