Support for container types

[RReverser]

Often, structs and enum contain types like Vec<SomeType>, HashMap<A, B> and so on. Currently there is no way to use displaydoc on such structs and enums altogether.

I suggest that there should be either a way to deal with iterable collections, or, simpler, a way to override display with custom expression for individual fields (e.g. like derive(Display) does in derive_more crate: https://jeltef.github.io/derive_more/derive_more/display.html).