make "minus sign" configurable

[oberhamsi]

I have found commits where the current minus-sign is intentionally hardcoded as it is, and as a constant.

i don't understand the reasoning for selecting this minus sign since for example in german we would rather show the smaller minus sign "-"

d3.locale would support selecting any character as a minus sign https://github.com/d3/d3-format#formatLocale

i can try a patch if this is something you want to make configurable. and if someone can give me a pointer where such a configuration option would best be placed (locale related?)