Add support for: WithMaxDepth, WithAppendSlice, WithKeyStringer

[shivamMg]

[peterbourgon]

Can you say a bit more why you want these changes? AppendSlice makes sense, but why the others?

[shivamMg]

I was trying to use the library with maps returned from YAML unmarshal. Most use cases arose from there.

• WithMaxDepth: Needed control over depth of nested key-value maps in YAML
• WithKeyStringer: In case of a nil map key, reflect would return "nil" string. My use case required it to be "null". Example.

I do understand that these might not be super-relevant for the library. Feel free to ignore the PR.

[peterbourgon]

Yeah, I'll probably add the MaxDepth option, and might add the AppendSlice option, but I think the KeyStringer bit is out of scope.