Handling of config structs with tagged struct fields

[hariso]

Ecdysis makes it possible to build flags from a struct, and in #1 we added support for parsing nested structs. We found that the following case is possible and needs to be properly handled:

type Config struct {
  DBConfig DBConfig `long:"database config"` 
}
type DBConfig struct {
  URL string `long:"db url"`
}

Nested structs are not flags on their own, they are merely a way to group flags. The goal of this issue is to clarify the usage of tags on nested structs (we can either forbid them, or allow them if we have a way to describe groups of flags).

[lovromazgon]

IIUC this issue is about adding a prefix to nested flags, correct? For example, these structs:

type Config struct {
  DBConfig DBConfig `long:"database"` 
}
type DBConfig struct {
  URL string `long:"url"`
}

... would result in the flag -database.url=foo, right?

[hariso]

@lovromazgon Yes, that's one of the options. Another is to just disallow that usage at all. My intention with this issue is to clarify which do we want to use and be consistent with that.