Add indexer to DynamicParameters so that C# 7 dictionary initializer syntax can be used

I've got some code that looks like this:

public class MyDynamicParameters : DynamicParameters {
    public object this[string key] {
        set => Add(key, value);
    }
}

Which lets MyDynamicParameters be initialized like this:

new MyDynamicParameters() {
    ["Id"] = 1,
    [Constants.MySpParamName] = "test",
}

It would be nice if this indexer setter could be introduced to DynamicParameters to make initializing this object quite clean, over a list of parameters.Add(key, value) calls.

I can take a shot at implementing this if you want - just let me know and I'll make a PR. One question if I do though: should I modify the SqlMapper.IParameterLookup.this[object name] interface member to include a setter (might be breaking for those who have implemented this?)

Locate DynamicParameters and the SqlMapper.IParameterLookup interface, then read how parameter names and values are currently added and exposed. Check the existing tests around DynamicParameters before deciding how the setter should interact with the interface. Done means C# 7 dictionary initializer assignments work without breaking existing implementations.