Add option to add namespaced attributes

[mdamien]

Ex: <text xml:space="preserve" bytes="1181">
Current workaround:

raw('<text xml:space="preserve" bytes="1181">'),
render(filecontents),
raw('</text>'),

related to #2

[mdamien]

In fact, lys already works with namespace attributes !

            L.text(**{'xml:space': 'preserve', 'bytes':'1181'}) / (
                render(filecontents)
            ),