Log debug output in helm.Config [#9]

Redacting KubeToken may not be sufficient, since it's possible that
someone would put secrets in Values or StringValues. Unilaterally
redacting those seems unhelpful, though, since they may be the very
thing the user is trying to debug. I've settled on redacting the obvious
field without trying to promise that all sensitive data will be hidden.

internal/helm/config.go

@@ -1,7 +1,9 @@
 package helm
 
 import (
+	"fmt"
 	"github.com/kelseyhightower/envconfig"
+	"io"
 )
 
 // The Config struct captures the `settings` and `environment` blocks in the application's drone
@@ -33,11 +35,17 @@ type Config struct {
 	Chart              string   ``                                   // Chart argument to use in applicable helm commands
 	Release            string   ``                                   // Release argument to use in applicable helm commands
 	Force              bool     ``                                   // Pass --force to applicable helm commands
+
+	Stdout io.Writer `ignored:"true"`
+	Stderr io.Writer `ignored:"true"`
 }
 
 // NewConfig creates a Config and reads environment variables into it, accounting for several possible formats.
-func NewConfig() (*Config, error) {
-	cfg := Config{}
+func NewConfig(stdout, stderr io.Writer) (*Config, error) {
+	cfg := Config{
+		Stdout: stdout,
+		Stderr: stderr,
+	}
 	if err := envconfig.Process("plugin", &cfg); err != nil {
 		return nil, err
 	}
@@ -54,5 +62,16 @@ func NewConfig() (*Config, error) {
 		}
 	}
 
+	if cfg.Debug && cfg.Stderr != nil {
+		cfg.logDebug()
+	}
+
 	return &cfg, nil
 }
+
+func (cfg Config) logDebug() {
+	if cfg.KubeToken != "" {
+		cfg.KubeToken = "(redacted)"
+	}
+	fmt.Fprintf(cfg.Stderr, "Generated config: %+v\n", cfg)
+}