Expose "save" event in Office.js across platforms

Office.js should expose save events that allow add-ins to take an action before the document actually gets saved.

This is especially an issue when using such platforms like Word in the browser, which auto saves very frequently.

For example, we want to ensure a document gets saved encrypted at all times, but still allows the end user to work with the unencrypted content of a document. Currently, as far as I know, there is no way for us to take any sort of action before a document gets saved (especially to the cloud). This is a major issue when working with sensitive data that should never be saved unencrypted to cloud storage.