How to define Namespace

e. g. xmlns:feeder="https://nononsenseapps.com/feeder"

``` xml
<?xml version="1.0" encoding="UTF-8"?>
<opml version="1.1" xmlns:feeder="https://nononsenseapps.com/feeder">
  <head>
    <title>
      Feeder
    </title>
  </head>
  <body>
```