Add support for graphviz edge "dir"

[bewing]

If the "dir" attribute is present in the edge, disable both arrows,
and selectively re-enable them based on dir.

[mojoaxel]

Could you please also add documentation for this feature. I think somewhere near here: https://github.com/almende/vis/blob/develop/docs/network/edges.html#L193

[bewing]

It appears the only DOT-specific documentation is at https://github.com/almende/vis/blob/develop/docs/network/index.html#L1568 -- I did add some examples to https://github.com/almende/vis/blob/develop/examples/network/data/dotLanguage/dotEdgeStyles.html -- perhaps just add some explicit language there specifying the mapping from DOT dir to vis.js arrows?

[mojoaxel]

perhaps just add some explicit language there specifying the mapping from DOT dir to vis.js arrows?

I didn't realize that this is a DOT format feature.

LGTM