support formatting complete graph in graphviz dot format for visualization

[fogti]

Although this capability wouldn't be that useful for production setups, because the data sets would be way too large, and even if graphviz could handle that, it would be hard to find any image viewer or such which could deal with output files... (this is something which I learned when I tried to visualize pijul change log graphs, because they also get big quickly)
But it would be really useful while debugging smaller graphs and algorithms modifying them. I would place such support into the indradb-lib behind a feature flag It should be placed into a plugin.

[ysimonson]

That makes sense, though it should probably go in a plugin to keep lib minimal.