Add support for reading from stdin and writing to stdout (+ associated API changes)

This is required for integrating Flowlessly with Firmament, which sends the graph via a pipe connected to stdin for the solver, and expects the solver output on stdout.

Attached patch changes the default input/output of Flowlessly to stdin/stdout (if the file name arguments are empty, which is the default). This also requires minor API changes to `readGraph()` and `writeFlowGraph()` in the `Graph` class.