#229 dot format heuristics for automata layout

[Flammie Pirinen]

This is a low priority RFE but I noticed when doing slides for a presentation that often you need to tweak settings of output dot code from hfst-fst2txt to get readable automata. It would be neat if the program made some educated guesses about potentially good layout of the automaton and wrote them to the header. And also did the necessary black magic to turn epsilons into the codes that can produce varepsilon in some dot outputs.