add support for embedding struct configuration with special "_" tag

[stevevls]

Issue #22

[stevevls]

First pass at the proposal to use _ as the special tag to embed structs. LMK what looks good and what doesn't.

[achille-roussel]

This is looking great 💥

[stevevls]

Awesome! I added a couple more test cases and squashed a couple of bugs. I also searched the project for anywhere we were doing field.Tag.Get("conf") to make sure that I wasn't missing any additional spots where we're inspecting the tag. I only come up with the two places: one in makeNodeStruct and the other in fieldPath. Let me know if there are any other places you can think of that need to be updated!