Allow building graph of all issues vs. issues with edges, perhaps with reachability from a given set of issues

[rick]

For some projects, just pulling in every encountered issue ("all issues mode"), whether there are depends/blocks edges between them or not, is feasible and makes sense.

For many projects, grabbing all issues could result in thousands of issues and an incomprehensible graph, in which case we may want to limit to only those issues which have depends/blocks edges between them ("edges only mode").

Regardless, we are likely to be focused on issues related to a given set of work, in which case it would be ideal to specify one or more issues as starting points for building our graph ("focused mode").

Also consider an option to not show closed issues in the graph (easy enough to do as the seam is already in the code).

Making these command-line flags and arguments seems like the right thing to do here.

[jameswhite]

I think the first thing is "ignore closed issues with no edges."