HTTP Requests with Precision parameter

[jaket466]

I've just started working with your library, and I've found it quite helpful and easy to use so far. However, I got to a point where I needed to change the precision used for the timestamps, and I ended up modifying the http_request function to support this. It essentially just adds the precision parameter to the query, like so:

Is this something you'd want to include in your library, or is there some other way of doing this I missed? I can submit a pull request with my changes if you'd like.

[orca-zhang]

Thanks for your liking influxdb-cpp and contribution. Nice enhancement you've made, and please feel free to submit the PR.