Implement 'xmlattr' filter

[flexferrum]

Implement 'xmlattr' filter according to the jinja2 specification:

Create an SGML/XML attribute string based on the items in a dict. All values that are neither none nor undefined are automatically escaped:

<ul{{ {'class': 'my_list', 'missing': none,
        'id': 'list-%d'|format(variable)}|xmlattr }}>
...
</ul>

Results in something like this:

<ul class="my_list" id="list-42">
...
</ul>

As you can see it automatically prepends a space in front of the item if the filter returned something unless the second parameter is false.

[vvish]

Please assign this task on me