Add support for HTML attributes

[oriolpiera]

Goal

This PR add support for insert attributes on specific html tags using dicttoxml_attributes and dicttoxml_content as a "reserved words" .

Example with on element

Input:

"name": { 
    "language": { 
        "dicttoxml_attributes": { 
            "id": "1", 
         }, 
         "dicttoxml_content": "This is a text", 
     } 
  }, 

Result:

<name>
    <language id="1">This is a text</language>
</name>

Exemple in a list

Input:

"name": [ {"language": { 
        "dicttoxml_attributes": { 
            "id": "1", 
         }, 
         "dicttoxml_content": "This is a text", 
     }},{"language": { 
      "dicttoxml_attributes": { 
       "id": "2", 
       },  
       "dicttoxml_content": "This is another text", 
       }} 
  ], 

Result:

<name>
    <language id="1">This is a text</language>
    <language id="2">This is another text</language>
</name>

Example with more than one attribute

Input:

"name":  {"language": { 
        "dicttoxml_attributes": { 
            "id": "1", 
            "quality": "45",
            "class": "header"
         }, 
         "dicttoxml_content": "This is a text", 
     }
}, 

Result:

<name>
    <language id="1" quality="45" class="header">This is a text</language>
</name>

[bfontaine]

This feature has been extensively discussed in #66.