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.
I believe we might want to make this
`if xpath_expr and format == 'xml'` just to make sure that we can't hit this if they didn't set `-f xml`. If they send an xpath filter on a show command and don't change the format to `-f xml`, then perhaps a third case for `if xpath_expr and format == 'text'`that would let the user know they can only do xpath filtering on xml format.