[tern addon] add optional callback to showType

In Firefox DevTools we are using the tern addon, and we need to be able to be notified once a popup is shown for testing purposes.  We have a [customized showType implementation](http://dxr.mozilla.org/mozilla-central/source/browser/devtools/sourceeditor/codemirror/tern.js#224) for this, but it would be great if we could contribute the change back to the addon.  I'd be happy to add a test for this, but I didn't see any existing tests for the tern addon so I've left that alone for now.