Usage:
>>> add_query_params('http://foo.com', a='b')
'http://foo.com?a=b'
>>> add_query_params('http://foo.com?a=b', b='c', d='q')
'http://foo.com?a=b&b=c&d=q'
The real implementation should be more strict, e.g. raise on the
following:
>>> add_query_params('http://foo.com?a=b', a='b')
'http://foo.com?a=b&a=b'