Might be helpful for issue actionshrimp/vim-xpath#1

lxml style "{}" namespaces didn't work for me, so complete the namespace
prefix instead

This code works, but ...
    modifies the loop variable instead of making a copy of results
    loops over the nsmap to find the namespace prefix, this could be
    slow if there are a lot of tags and a lot of namespaces

This commit might be helpful for Issue #1.  Doesn't solve it, but I found it much nicer to get namespace completion on prefix instead of curly backet namespaces.