Report on missing translations suitable for use on GitHub

[hadley]

After running po_update() it would be nice to be able to generate a markdown block suitable for use on GitHub. I think this would be a check list for each language that cc's the translator (using the GitHub username from the .po file), listing all words that need updates.

Maybe something like this:

Missing translations for:

* [ ] fr (cc @frenchtranslator)
    ```
    msgid "Search for"
    msgid "Version"
    ```

* [ ] de (cc @germantranslator1, @germantranslator2)
    ```
    msgid "Skip to contents"
    ```

Complete translations: jp, po.