Add callback attribute to execute after file is parsed

[AnnaKalkanis]

Addresses the second point in issue #21. A single callback function can now be passed into the directive as an attribute. The callback takes one parameter, which will be the event which triggered the file to be parsed (ie. change in file, change in separator...etc). The callback is executed once the file has been successfully parsed and the assignment of the parsed file to the 'result' variable has completed.

[samanthacatania]

I tried this commit and the callback was being called from strange places. I changed the argument callback to something else (in my case csvCallback) and the code started working as expected.

[JohnTheBeloved]

Can you kindly merge(if all is fine), Also need this feature asap

[bahaaldine]

Thanks for your help folks