core: allow custom scope help in prompt

[blackxored]

Please allow for custom-configs to set prompt help descriptions. I think it would be handy to be able to have configs specify descriptions when using a fixed set of scope values.

[marionebl]

How would you image this to look like? Ideally, I'd like to come up with something that works for all the *-enum types

[blackxored]

I took what might have been the hard approach in #150, allowing config writers to specify that.