Define the concept of an output formatter

The general idea here is that I want to be able to output my
hand-computed graphs in a number of different ways -- at least Flo and
Chisel to start.
Use libflo for circuit graphs

I realized that I'd be duplicating a whole bunch of libflo in order to
deal with all the graph stuff so instead I just went ahead and used
it.  Note that this is going to require a whole bunch of changes to
the actual implementation of libflo, but I guess I'll have to live
with that for now... :).