Add clickAction to sankeyNetwork

[Atrebas]

In forceNetwork, clickAction allows to trigger an action when a node is clicked. Would it be possible to add this feature to sankeyNetwork? What I have in mind is to display the elements that belong to a node when it is clicked (e.g. in a table below).

Here is a procedure that seems to be ok (copy/paste from forceNetwork):

## In sankeyNetwork.js
# line 241, above the dragmove function

function click(d) {
    return eval(options.clickAction)
}

# line 157, in var node before .call([...])

.on("click", click)

## In sankeyNetwork.R
# line 135, at the end of options

clickAction = clickAction

# line 79, at the end of the sankeyNetwork arguments

clickAction = NULL

# line 44, in the doc
#' @param clickAction character string with a JavaScript expression to evaluate when a node is clicked.

Thanks.

[Atrebas]

Just saw there is already PR #139 for this, but there are some conflicts.