Make serverlog() work with rotating logs
Make server.log configurable

Expose the file that serverlog() uses as a SERVER_LOG option in
clicraft.conf.
Restore SERVER_JAR if download fails
Make stripping leading command slashes optional

Add an option, CMD_STRIP_SLASHES, which configures whether
`clicraft cmd` will strip the leading slash of a command before
passing it to the server console.
Add STOP_CMD config option

Expose the command that's sent to the server console when running
`clicraft stop`.
Add missing options to default clicraft.conf