Improve output format of dependencies graph

[dlesnoff]

Currently, it is possible to output the dependencies with a graph representation using GraphViz.
See: https://github.com/nim-lang/atlas/blob/master/src/atlas.nim#L446

« Ideally Atlas would output a graph representation that is not only useful for GraphViz. Maybe in JSON and then some external tool would render that in the browser. » Araq from this forum post.

[Araq]

The problem is not that the graph representation is bad (though it might be), the problem is that we don't dump out a graph representation that is more common and flexible than the .dot-file format.