Reorder VDR channels.conf

This is a new feature that allows to reorder VDR channels list, before sending it to XBMC, with an external script or program. The feature is only enabled if the configuration parameter ReorderCmd is set, otherwise it should not impact on the currently implemented functionality.

I've tested it with this https://github.com/AndreyPavlenko/chorder script and below configuration:

 ReorderCmd = /opt/chorder/chorder -X /opt/chorder/examples/channels.conf.exclude -F /opt/chorder/examples/channels.conf.fields -t /opt/chorder/examples/channels.conf.template