Initialize run.Configs in the NewSTEP functions [#67]

This fixes the run package's leaky abstraction; other packages no longer
need to know or care that run.Config even exists.

Note that since the various Steps now depend on having a non-nil pointer
to a run.Config, it's unsafe (or at least risky) to initialize them
directly. They should be created with their NewSTEPNAME functions. All
their fields are now private, to reflect this.

internal/run/config.go

@@ -1,13 +1,22 @@
 package run
 
 import (
+	"github.com/pelotech/drone-helm3/internal/env"
 	"io"
 )
 
-// Config contains configuration applicable to all helm commands
-type Config struct {
-	Debug     bool
-	Namespace string
-	Stdout    io.Writer
-	Stderr    io.Writer
+type config struct {
+	debug     bool
+	namespace string
+	stdout    io.Writer
+	stderr    io.Writer
+}
+
+func newConfig(cfg env.Config) *config {
+	return &config{
+		debug:     cfg.Debug,
+		namespace: cfg.Namespace,
+		stdout:    cfg.Stdout,
+		stderr:    cfg.Stderr,
+	}
 }