Add Clonk Center

This program now supports crawling the download page of a Clonk-Center archive. This allows it to download even more items that might disappear someday.

From a programming viewpoint it is now easier to add a new source as you only have to implement the following interface for the item struct (and add json struct tags)

type Archivable interface {
	GetDownloadLink() string
	GetAuthor() string
	GetName() string
	GetSourceName() string
}