Use a library to generate dot files instead of string concatination

[MartijnBraam]

Like https://github.com/alexandresalome/graphviz

[clemens-tolboom]

As #45 was reverted by #48 this is reopened.

[clemens-tolboom]

As PR #45 was reverted by PR #48 this needs more work:

• [ ] Testing graphviz output as PR #45 broke our tool completely
• [ ] Check with https://github.com/clue/graph and its toolset
• [ ] Check with https://github.com/alexandresalome/graphviz PR https://github.com/alexandresalome/graphviz/pull/7