adding custom img attrs to image_tags

I did not create a custom TestCase for it. I also trust the template author not add ``width``, ``height``, ``alt``, or ``src`` custom-attributes, which would also get added to the ``<img>`` tag as duplicates.

I could handle syntax errors better (for example, not raising an error on any duplicate attr-names including the default ones above), but this should work for basic usage.

This addresses #284
I'd like to avoid adding mutable default values to keyword arguments. This isn't a merge-blocker though.