Add line numbers to Halstead results

[devdanzin]

I'm currently adding a feature to wily that allows annotating source code with radon metrics. Cyclomatic Complexity results already have lineno and endline entries, making annotation trivial. However, Halstead metrics don't have this information.

We are able to work around this issue by subclassing HalsteadVisitor and HCHarvester to add line numbers, but thought maybe this feature would be accepted upstream.

If there's interest, I can submit a PR once #247 either lands or is rejected.