Executes the body if the condition is true

@param {String} args The arguments to the command
@param {Function} define
@constructor
function IfCommand(args, define) {
    if (util.endsWith(args.toLowerCase(), ' then')) args = args.slice(0, args.length - 5).trim();
    else throw new SyntaxError('IF has no THEN');

    var parsed = new statements.ArgumentStatement(args, {
        separator: false
    }, define);

    this.condition = parsed.args[0];
    this.block = define({
        start: 'IF',
        then: 'ELSE',
        end: ['ENDIF', 'RETURN']
    });
}