Add support for capturing source position/locations when parsing

[jeff303]

I've been looking for a JSON parser in Clojure that can capture the character positions (from the source) for parsed structures. See, for example, the zserge/jsmn project.

If this seems like a useful thing to have (even in an experimental state), I can work on it.

[jeff303]

Since Jackson appears to be used under the covers, I suspect this is feasible.

[dakrone]

I think this sounds useful and interesting, please feel free to work on it!