Allow any expression after 'track by '

[fernandogmar]

Hi @kamilkp

I am using your library in one of my projects, I have found that is not possible to ' track by SOME_EXPRESSION' that can be possible normally using ng-repeat.

I hope this change can help you or any other with the same use case...

BTW you could find more information about what I was trying to achieve and you could do with this change ;) https://www.bennadel.com/blog/2920-using-a-compound-track-by-expression-with-ngrepeat-in-angularjs.htm