generate connection numbered icons

[alixander]

now that we can do icons for connections (#12), we should make this common use case more convenient. To find icons for 1, 2, 3, ... would be ridiculous.

a -> b: {
  icon: 1
}
b -> c: {
}

[bo-ku-ra]

avoid similarity to "--force-appendix".

[alixander]

#2356