feat: Auto-detect term for coloring helm-diff output

Since helm-diff has added an ability to auto-detect the term to decide if it should output with color or not, helmfile had been defaulted to no-color.
This resoloves that, by adding a term-detection logic that is same as helm-diff.

As a part of this work, I have also implemented a new global flag `--color`, which is used for forcing color without relying on the term-detection logic implemented in helmfile or explicitly setting the HELM_DIFF_COLOR envvar. I hope it is useful for folks.

Ref https://github.com/roboll/helmfile/issues/2043

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

pkg/app/diff_test.go

@@ -93,6 +93,10 @@ func (a diffConfig) SuppressDiff() bool {
 	return a.suppressDiff
 }
 
+func (a diffConfig) Color() bool {
+	return false
+}
+
 func (a diffConfig) NoColor() bool {
 	return a.noColor
 }