Simple TFL Underground query node (issue #84)

Pull request covering the very simple case detailed in issue #84.

One thing that is missing is the icon, however, in order to get this we have to register again to obtain the logo...not sure what we want to do here. This will be the same problem with the bus and bike nodes.

NB - the line can only be chosen in the ui and defined via the node configuration. A future enhancement would be to enable this to be passed through on a previous msg. However, that is an enhancement. I think this node is ready to go as is.