Feature request: specify relative location for edgelabel along the edgelength

[hdavid16]

PR #186 allows placing edge labels on the middle of an edge, whether it be curved or straight. It would be nice to have a kwarg for gplot that will allow the user to specify the position in the range 0 to 1 for the relative location along the edge (e.g., 0.5 = midpoint, 0 = startpoint, 1 = endpoint). This can easily be done by changing the 0.5 to the parameter value in the section of gplot where the edgelabels are placed.