Enable helmfile-diff to pass the output format to helm-diff (#1784)

* tests: fix vagrant test run

* feat: added an option to specify the different diff output format

* renamed diff-output to output

* renamed diff-output to output

Co-authored-by: Andrey Tuzhilin <andrey@zelf.co>

pkg/app/diff_test.go

@@ -32,6 +32,7 @@ type diffConfig struct {
 	suppressDiff      bool
 	noColor           bool
 	context           int
+	diffOutput        string
 	concurrency       int
 	detailedExitcode  bool
 	interactive       bool
@@ -94,6 +95,10 @@ func (a diffConfig) Context() int {
 	return a.context
 }
 
+func (a diffConfig) DiffOutput() string {
+	return a.diffOutput
+}
+
 func (a diffConfig) Concurrency() int {
 	return a.concurrency
 }