Update MechJebModuleDockingGuidance.cs

Ever try to build something truly monumentally large in orbit? Lots of docking... ultimately you'll need a docking AP to get things done. Unfortunately, the MechJeb docking AP doesn't always play nicely with the KSP docking port targeting system. Specifically, if the docking port is on a huge construct, the MechJebModuleDockingAutopilot.targetSize may be larger than the ~200m maximum KSP sets for a docking port target lock. If this happens, MJ never docks, because it loses its target lock on the docking port before it finishes backing up to get outside of the distance specified by targetSize. I added a new radio button and text box to allow custom target sizes to be used, bypassing MJ's auto-calculated starting distance. Two files needed changing to make this feature work: MechJebModuleDockingGuidance.cs, and MechJebModuleDockingAutopilot.cs.