Add support for IPv6.

This pull request adds the tuple `{ipfamily, inet|inet6|inet6fb4}` to the env vars, returns it from `raven:get_config/0`, and adds a call to `httpc:set_options/1` in `raven:capture/2`.

I also took the liberty of making a few related simple changes, like modifying `raven:get_config/0` to return a `#cfg{}` record (to support future configuration enhancements), and exporting `raven:get_config/0` for debugging assistance.