Fix slice deep map

[ayratsa]

Implements deep mapping of nested slices of structs
Resolves the issue
Actually a copy of this PR

[EronWright]

The implementation seems to cover deep copy of slices, but not of other types (notably maps, to fix #12 and #40). I would advocate for a more comprehensive fix, or for renaming the config item to be more specific (DeepSlices).

[loicalleyne]

Any news on this? Does this replace or complement mapstructure.RecursiveStructToMapHookFunc?

[jphastings]

I think this is stalled because @mullerch created the original PR (where @ayratsa has kindly moved it over, after the project's change of hands).

Perhaps @sagikazarmark would care to weigh in? I'd be happy to make any needed changes if @ayratsa isn't available.