add use-external-data-flow-diagram argument

[CMon]

add use-external-data-flow-diagram to replace the generated diagram with a custom one.

If one has a larger threat model the graphviz version is quite ugly.

[CMon]

I have no clue on how to fix the "security issue" since it is essential that the filename comes from a variable

[softScheck]

This is exactly what #71 is supposed to do. The flags are already there, but get overwritten by treagile, this should be changed.

[ezavgorodniy]

@CMon I'd perhaps agree that approach at #71 would allow to achieve the same results