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/config.go

@@ -10,6 +10,8 @@ import (
 	"gopkg.in/yaml.v2"
 )
 
+var cfgFilenames = []string{".genqlient.yml", ".genqlient.yaml", "genqlient.yml", "genqlient.yaml"}
+
 // Config represents genqlient's configuration, generally read from
 // genqlient.yaml.
 //
@@ -109,6 +111,17 @@ func ReadAndValidateConfig(filename string) (*Config, error) {
 	return &config, nil
 }
 
+// ReadAndValidateConfigFromDefaultLocations looks for a config file in the
+// current directory, and all parent directories walking up the tree. The
+// closest config file will be returned.
+func ReadAndValidateConfigFromDefaultLocations() (*Config, error) {
+	cfgFile, err := findCfg()
+	if err != nil {
+		return nil, err
+	}
+	return ReadAndValidateConfig(cfgFile)
+}
+
 func initConfig(filename string) error {
 	// TODO(benkraft): Embed this config file into the binary, see
 	// https://github.com/Khan/genqlient/issues/9.
@@ -126,3 +139,35 @@ func initConfig(filename string) error {
 	}
 	return nil
 }
+
+// findCfg searches for the config file in this directory and all parents up the tree
+// looking for the closest match
+func findCfg() (string, error) {
+	dir, err := os.Getwd()
+	if err != nil {
+		return "", errorf(nil, "unable to get working dir to findCfg: %v", err)
+	}
+
+	cfg := findCfgInDir(dir)
+
+	for cfg == "" && dir != filepath.Dir(dir) {
+		dir = filepath.Dir(dir)
+		cfg = findCfgInDir(dir)
+	}
+
+	if cfg == "" {
+		return "", os.ErrNotExist
+	}
+
+	return cfg, nil
+}
+
+func findCfgInDir(dir string) string {
+	for _, cfgName := range cfgFilenames {
+		path := filepath.Join(dir, cfgName)
+		if _, err := os.Stat(path); err == nil {
+			return path
+		}
+	}
+	return ""
+}