file: add DefaultFormat option around equal sign (#164)

Add option to have padding spaces around equal sign when PrettyFormat is disabled #161

eg:

> ini.PrettyFormat = false
>         ini.DefaultFormatLeft = "  "
>         ini.DefaultFormatRight = " "

file.go

@@ -227,7 +227,8 @@ func (f *File) Append(source interface{}, others ...interface{}) error {
 }
 
 func (f *File) writeToBuffer(indent string) (*bytes.Buffer, error) {
-	equalSign := "="
+	equalSign := DefaultFormatLeft + "=" + DefaultFormatRight
+
 	if PrettyFormat || PrettyEqual {
 		equalSign = " = "
 	}

ini.go

@@ -48,6 +48,10 @@ var (
 	// at package init time.
 	LineBreak = "\n"
 
+	// Place custom spaces when PrettyFormat and PrettyEqual are both disabled
+	DefaultFormatLeft = ""
+	DefaultFormatRight = ""
+
 	// Variable regexp pattern: %(variable)s
 	varPattern = regexp.MustCompile(`%\(([^\)]+)\)s`)