Fix string escaping

[Merovius]

If a string contains quote-characters, they don't get correctly
escaped, causing syntax errors in the resulting graphviz file.
This quotes the string using strconv and then removes the
quote-characters added by that.

[bradleyjkemp]

Fails because tests currently expect strings to be quoted. I think you just need to append and prepend "\"" to the string you've got though and then this should be good to go. Thanks :)

[Merovius]

Hm, I think I should have a 2nd look, though. If I do that, the tests pass, but the svg-output looks like this which seems suboptimal. I updated the PR. The tests pass (at least locally) and the output looks like a go string literal, which is probably what we want.
But I do feel a tiny bit dirty about the resulting code…

[bradleyjkemp]

Hah yeah the double strconv.Quote is a bit surprising. Can you just add a quick comment to explain this (i.e. that the first Quote makes the string look like a go literal and then the second one escapes it for graphviz) and then I'll merge?

[Merovius]

Done.