plugins.scraper.prefix config var added.

This config var set a prefix to use when printing a scrap’s results. Printing without a prefix may be used to make botaniko write messages to other users. For example, using [Pastebin](http://pastebin.com/raw.php?i=YHFyNBu1), you can make botaniko write anything you want. Just write the message on Pastebin, then create a scrap on that page which captures everything (`(.*)`), and you can now trigger the message using `botaniko: scrap <name>`. Note that using a prefix does not prevent mentions (e.g. `… @somebody …`), but it makes clear that this is the output of a scrap, not a normal message.