Provide an option to handle the data event

It would be nice if users can create their own custom functions to handle the data event. 
I'm executing a command that checks my php files for syntax errors. 
I would like to be able parse each output as it comes and check if there are no errors, otherwise exit the process.

## Example
```
---
    exec: {
        test: {
            cmd: "find ./component -name '*.php' -exec php -l {} ;",
            
            onOutData: function (data) {
                console.log(data);
                
                if (data.match(/Errors parsing|PHP Parse error/g)) {
                    process.exit(1);
                }
            },

            onErrData: function (data) {
                console.log(data);
                
                if (data.match(/Errors parsing|PHP Parse error/g)) {
                    process.exit(1);
                }
            }
    }
---
```