Question: postMessage API - App -> inAppBrowser Window

Hi everyone! I'm asking the question here since the link to sign up to Cordova Slack doesn't seem to work: http://slack.cordova.io and I couldn't find another way to sign up to the Slack channel.

**Context**
The postMessage API has been implemented (https://github.com/apache/cordova-plugin-inappbrowser/pull/362) and is documented here: https://github.com/apache/cordova-plugin-inappbrowser

**What have I tried to do?**
So far I was only able to find documentation for the direction from inside the inAppBrowser window to the context running the inAppBrowser - most of the case the native/cordova app (i.e. inside -> outside). Is it also possible to send a message from the outside to the inside. Basically, in the same direction that `executeScript()` and `injectCSS()`work.

**What would I like to achieve?**
Currently, I'm launch the inAppBrowser, navigate to a specific URL which is also under my control on which I have registered an event handler `window.addEventListener("message",...)`. Now, I would like to post a message from the app code to the inAppBrowser instance (i.e. the browser window that is on this specific url) once a event happens on the native device. Is this possible?

Start with the plugin documentation and pull request #362, then compare the documented postMessage direction with executeScript() and injectCSS(). Confirm whether app-to-inAppBrowser messaging is supported; if not, define the needed behavior and documentation before implementation.

Written by the indexing model from the issue text.