support for namespaced Path Elements

Hi all,
in [ParseElem]( https://github.com/google/gnxi/blob/master/utils/xpath/xpath.go#L210) the code doesn't allow for a pathElem to have a `:` char inside of it.

I wonder if this should be allowed to make the path to look life follows: `/root/ns:leaf`. Some vendors [use](https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/content/en/us/td/docs/ios-xml/ios/prog/configuration/172/b_172_programmability_cg/grpc_network_management_interface.html.xml#ariaid-title8) namespaced paths in their gNMI implementations, and I am not sure if this violates the gNMI spec or not.

@robshakir do you have an opinion if the gNMI paths like `/root/ns:leaf` are valid and should be supported?