Add simple implementation of CommandCallable

[boformer]

It would be nice to have a simple abstract implementation of CommandCallable (like SimpleDispatcher implementation for Dispatcher).

The abstract class provides the basic functionality that is sufficient in many cases. The only method it should not implement is the call method. A command class extending the abstract class would look like this:

public class SendCommand extends AbstractCommand {
    public SendCommand() {
        super(
                "simplemail.write", // permission
                "Send a mail", // description
                Texts.of("Send a server mail to a player."), // help
                "<player> <msg>"); // usage
    }

    @Override
    public boolean call(CommandSource source, String arguments, List<String> parents) throws CommandException {
        // Command stuff...
        return true;
    }
}

This is the proposed abstract command class:

public abstract class AbstractCommand implements CommandCallable {
    private final String permission;
    private final String shortDescription;
    private final Text help;
    private final String usage;

    public AbstractCommand(String permission, String shortDescription, Text help, String usage) {
        this.permission = permission;
        this.shortDescription = shortDescription;
        this.help = help;
        this.usage = usage;
    }

    @Override
    public boolean testPermission(CommandSource source) {
        return source.hasPermission(this.permission);
    }

    @Override
    public String getShortDescription(CommandSource source) {
        return this.shortDescription;
    }

    @Override
    public Text getHelp(CommandSource source) {
        return this.help;
    }

    @Override
    public String getUsage(CommandSource source) {
        return this.usage;
    }

    @Override
    public List<String> getSuggestions(CommandSource source, String arguments) throws CommandException {
        return Collections.emptyList();
    }

}

[gabizou]

If you do want to write a PR for this, make sure to use checkNotNull in the constructors.