Ineffective `weights` argument in `fortify.network`

Looking at PR #25 in detail, @jcfisher did not port the `weights` argument from `fortify.network` to `fortify.igraph` — with good reason, since the argument does nothing.

The argument is described as follows:

https://github.com/briatte/ggnetwork/blob/78f575bdcf4b9351e3c019835d892c68f802f830/R/fortify-network.R#L13-L16

And similar information is listed under 'Edge weights' in the vignette.

The problem is that there is no `weight` argument in `sna::gplot.layout.kamadakawai`, although its code calls the third column of the edge list at one point (line 754 below), with reference to an undocumented `edge.val.as.str` argument:

https://github.com/cran/sna/blob/38970cdcfee061c5ca237b885290738f24e0f3ef/R/visualization.R#L747-L758

It might thus well be that `sna` 'silently' weights the edge list in this case, although it is not very obvious (to me) how that happens, and whether users can provide `sna::gplot.layout.kamadakawai` (or similar layout algorithms that accept weighted edges) with a `network` object and expect edge weights to be taken in to account.

In all circumstances, contrary to what the documentation says, `fortify.network` does _not_ currently take those edge weights into account. Edge weights are added to the edge list, but discarded right after that happens:

https://github.com/briatte/ggnetwork/blob/78f575bdcf4b9351e3c019835d892c68f802f830/R/fortify-network.R#L145-L154

For edge weights to be taken into account by `fortify.network`, (1) I need to understand better whether the code I highlighted from `sna` is indeed a weighting technique, and (2) I need to pass the weighted edge list to the layout calculation, i.e. I need to compute `edges` before computing the `layout` matrix, and use that (`edges`) in the layout computation.

Checking our [_R Journal_ article](https://journal.r-project.org/archive/2017/RJ-2017-023/RJ-2017-023.pdf) revealed that `geomnet` seems to have a solution to that issue:

https://github.com/sctyner/geomnet/blob/ca44ee77d59f6532b7ab29fcea8c724c5831f4c1/R/stat-net.r#L82-L109