Added XML output and Xpath filtering - 2nd attempt

I added the ability for the user to specify XML output using --format xml at the command line. I also added the optional use of xpath expressions in the commands file.

An example line from the commands file might look like this -

show route 2600::/64 % //rt-entry

The op command is separated from the xpath expression by a '%' character.
This one's pretty nit-picky, but spaces should be around any operator that isn't a parameter being set in a function call/definition. (PEP8 C0326)