add xml namespace support

[marc-portier]

during parse we should be able to specify namespace-to-prefix mappings that can be used for actual member-finding later on

suggested way of working:

#input
xml = '<root xmlns="https://example.org/ns/default" xmlns:pfx="https://example.org/ns/other"><a>1</a><pfx:a>2</pfx:a></root>'
# can be inspected through
xdict=parse(xml, dict(ns1="https://example.org/ns/default", ns2="https://example.org/ns/other"))
assert int(str(xdict['ns1:a'])) == 1
assert int(str(xdict['ns2:a'])) == 2

[marc-portier]

While reading up https://github.com/bramstein/xsltjson
there seems to be something like the Badgerfish convention that is guiding this kind op stuff.