Add a configurable "edit button"

[Klortho]

Similar to "fork me on github" that you see on a lot of sites. This could facilitate people suggesting comments. Not sure how to make it generally configurable, so that it works with any kind of backend, but the idea is that it should take the user to a page where they can either create an issue in an issue tracker, or actually edit the original document and have it create an automated pull request.

Part of the motivation for this is that Debbie Lapeyre mentioned several times at JATSCon 2013(4) that they don't get enough examples. One reason for that might be that people don't know how to submit examples, or even that they are solicited.