Marshal and Unmarshal for structs

[trobro]

Uses json.Marshal() and json.Unmarshal() for intermediates to handle structs and Marshaler/Unmarshaler interfaces.

Now ignores comment tags on struct fields. Might need more investigation of consequences.

Edit: the comment tags on struct fields are back in.

Fixes #29
Fixes #31
Fixes #34

[trobro]

The yggdrasil-go project is using comment tags on struct fields:

https://github.com/yggdrasil-network/yggdrasil-go/blob/41b4bf69cfa7b0a2e0f7904867e49160f9283cea/src/config/config.go#L30