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.
if the user is doing an xpath filter on the command, perhaps we could show them in the output of the command what they are filtering to? For example, if they did 'show route 10.0.0.0/24 % //rt-entry' would show: "> show route 10/24 | display xml | no-more", but perhaps could show "> show route 10/24 | display xml | no-more % //rt-entry". 

I'm torn on this, since they wouldn't then be able to drop it into Junos, but the command printed to them is also not actually what they are doing, since it excludes any information about the xpath filtering. I'm leaning towards showing them the xpath-filter, so that the information is complete. I lean this way since the people likely to be doing xpath filtering are already going to understand that they can't drop that command into Junos and expect it to work.