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
}

Add more items

• Log failed items in extra file with error message. This will create a file that states which items might be missing and why
• Add additional items that should also be archived. Some items weren't linked on ccan.de (e.g. english versions of the games and freeware keys), but they are now added as they are important for the archive to be complete.
• Some cosmetic fixes to the readme - some text issues that would make the rendered readme file look ugly

Small fixes

This release includes small fixes:

• Typos
• Increased timeouts for downloads so they don't fail after 10 minutes

Add password

This release adds the password for the Clonk Endeavour developer mode to the README.md file in the zip file.

Initial release

This release includes the downloader that creates the result.zip file.