Reload graph from previous run

[jcoupey]

The graph is currently loaded from the osm.pbf file using osm4routing: https://github.com/Tristramg/roads-from-nd/blob/master/src/main.rs#L68 ; so the same parsing work is done on each execution.

It could be nice to be able to write down the node and edges files produced by osm4routing. The next execution with different parameters or start node might then get a significant speed-up by only loading the graph in memory from those files.

Another way of doing this would be to just provide an options to load from those files rather than from osm.pbf (and let users generates the files themselves using osm4routing).