Offer the opportunity to specigy position of nodes

[WenyinWei]

Hello all,

I have made Springy able to specify the position of Nodes, which may be anticipated for a long time.

Please see a demo in my personal website (You can also comment there).

Welcome your edit and I am not a js maintainer, so you can just edit if the code style does not fit the original one without informing me.

[WenyinWei]

Any response?