adding 'generateAggregate' method

- enable aggregate.js/css files to be generated fresh
- support sending generated string to through callback

This change will enable live reload of changes to *.js and *.css files
in packages by changing
https://github.com/linnovate/mean/blob/master/server/config/express.js#L
115 to:

        mean.generateAggregate('js', (process.env.NODE_ENV ===
'production'), function(s){
            res.send(s);
        });

Instead of:

        res.send(mean.aggregated.js);