Toggle-able fuel lines

[Panzerspartan]

With the way fuel lines work at the moment, in order to get fuel to flow a certain way (ie. from drop tanks into a spaceplane) you have to either design the vehicle with the fuel flow in mind or make a fuel line maze. Why not just make it possible to shut fuel lines on and off (valves) either through action groups or manually?

[LethalDose]

I'd also like to see a tweakable option for what fuel flows where (e.g. liquid fuel can flow through a duct, but not oxidizer).

[Kaos]

I'd like more a way for Kerbonauts on EVA to add and remove fuel lines; this in some sense includes the orignial idea.