Feature: Adds formatting of yaml output

The format produced by `gopkg.in/yaml.v2` produces output while valid
is not easy to read especially for arrays.
Adds formatter to handle formatting the marshalled yaml content.
Applies similar logic used by `encoding/json.Indent`.

Resolves: #25

formatter.go

@@ -0,0 +1,36 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+
+	yaml "gopkg.in/yaml.v2"
+)
+
+func format(context interface{}) (string, error) {
+	out, err := yaml.Marshal(context)
+	if err != nil {
+		return "", fmt.Errorf("error printing yaml: %v", err)
+	}
+	var dst bytes.Buffer
+	// add error checking if indent expanded to ever return an error
+	_ = indent(&dst, out)
+
+	return dst.String(), nil
+}
+
+func indent(dst io.ByteWriter, src []byte) error {
+	for _, c := range src {
+		switch c {
+		case '-':
+			_ = dst.WriteByte(' ')
+			_ = dst.WriteByte(' ')
+			_ = dst.WriteByte(c)
+
+		default:
+			_ = dst.WriteByte(c)
+		}
+	}
+	return nil
+}

yaml.go

@@ -417,11 +417,11 @@ func toString(context interface{}) (string, error) {
 }
 
 func yamlToString(context interface{}) (string, error) {
-	out, err := yaml.Marshal(context)
+	outStr, err := format(context)
 	if err != nil {
 		return "", fmt.Errorf("error printing yaml: %v", err)
 	}
-	outStr := string(out)
+
 	// trim the trailing new line as it's easier for a script to add
 	// it in if required than to remove it
 	if trimOutput {