added support for integrating selected data with plotly

[atitus]

If using Plotly, one can use the Box Select or Lasso Select tool and select data in a scatterplot. The area under the curve will be shaded, and the integral will be printed.

Here is a simple example program that illustrates the feature:

g = gdots(fast=False, integrate_selected=True)
for t in arange(0,10,0.01):
    g.plot(t, cos(t))

I have not tested it with gcurve(fast=False, integrate_selected=True). Because this also produces a Plotly scatterplot, it's possible that the data can be selected in this case too. Plotly only shows the selection tools for a scatterplot. Thus, this feature won't work for a default gcurve().