igraph 2 planning

See also 2.0 milestone: https://github.com/igraph/rigraph/milestones/2.0

Opportunities to make breaking changes

• Visualization in igraph 2
• More consistent and predictable interface, see e.g. https://github.com/igraph/rigraph/issues/622, https://github.com/igraph/rigraph/issues/526, and graph generators being inconsistent in whether they create directed or undirected graphs