In VS 2017 RC, navigate to Tools --> Options --> Projects and Solutions --> Web Package Management --> External Web Tools. Note that the 4 icons (Add, Remove, Up Arrow, Down Arrow) used here are aesthetically different than the icons used in Tools --> Options --> NuGet Package Manager --> Package Sources.
In VS 2017 RC, navigate to Tools --> Options --> Projects and Solutions --> Web Package Management --> External Web Tools. Note that the 4 icons (Add, Remove, Up Arrow, Down Arrow) used here are aesthetically different than the icons used in Tools --> Options --> NuGet Package Manager --> Package Sources.