Make --output optional

[Michael Haeuslmann]

All commands, including those producing binary output (e.g. .png) can output to stdout directly. This would allow for one line calls like

bin/dephpend uml src | display

instead of having to write it to an intermediate file.

[Michael Haeuslmann]

I will stop using issues for my personal feature requests because it makes the project look more broken than it is 😅