Httpie: Fix XML formatter

Issues

The existing implementation of XML formatting suffers from multiple issues:

• [ ] Namespaces get messed up #389
• [ ] Processing instructions get lost #415
• [ ] Potentionaly vulnerable #384
• [ ] Comments get lost
• [ ] Doctype gets lost

Possible Solutions

1. Find/implement a robust XML formatter
2. Remove XML formatting all together #394
3. Only format simple XML without comments, processing instructions, doctype, and namespaces.
   

   Example

Before:

<?pi data?>
<!-- comment -->
<root xmlns='namespace'>
   <element key='value'>text</element>
   <element>text</element>tail
   <empty-element/>
</root>

After:

<ns0:root xmlns:ns0="namespace">
    <ns0:element key="value">text</ns0:element>
    <ns0:element>text</ns0:element>tail
   <ns0:empty-element />
</ns0:root>