cmd/schema: add exclude labels flag to schema scanner

[adamstruck]

depends on #194

[kellrott]

The string based field negation (O.query().V("vertex1").fields("-symbol") seems like a new pattern in the JSON path that we haven't used before or may not be needed anywhere else. I don't have a better idea (using something like gripql.neq seems weird as well), but it seems a bit of an exception to regular behavior.

[adamstruck]

It is a new pattern specific to this method only. I thought that this was fairly standard syntax for field selection / negation, but am open to alternatives. We could base this off the project method from mongo: O.query().V("vertex1").fields({"symbol": 0}) and O.query().V("vertex1").fields({"symbol": 1})

[kellrott]

I guess we can test out the string based negation, and change it later if it isn't working for us.
👍