surfacing the string based input/output handling that Vulcanize now supports

I was going to redo the patch we had for making vulcan do string based input/output handling, but it turns out the new code already (almost) has the support baked in. This PR adds documentation that surfaces how this functionality can be used, and renames the output handling from `outputSrc` to `outputHandler`, since the option is used as a callback, so "Src" seemed the wrong suffix for the option name.