Add is_metadata kwdict flag and fix #865

[GiovanH]

You may want to edit the implementation of pseudo_kwdict? I wasn't sure what the self.extension check was designed to fix but I didn't want to cause a conflict with that behavior.

[mikf]

So Message.Metadata got removed (56a8968), which makes this PR rather superfluous.

The metadata post processor (if it should still be called that) now has a filename and event option, which allows for a better functionality than Message.Metadata and bypost ever had:

        "postprocessors": [
            {
                "name": "metadata",
                "event": "post",
                "filename": "{id}.json"
            }
        ]

It is also planned for 2.0.0 to have "event": "post" with a reasonable filename be the default for all extractors, so that --write-metadata or "postprocessors": [{"name": "metadata"}] will do the "right" thing without the need to specify any extra options.