formatter: add the ability to dump attrs in xml entities

formatter: add the ability to dump attrs in xml entities

xml entities may have attrs assigned to them. Add the ability
to set them. A usage example:

formatter->open_array_section_with_attrs("container",
     FormatterAttrs("name", "foo", NULL));

This will generate the following xml entity:
<container name="foo">

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>