Allow multiple column unique keys

#307 
I think i go it. This PR can handle ID combined with multiple values from one row - 1::context we need to break it and iterate over the exploded string and return the value of the row at a current position. In the example 1::context the first key will 1, the second key will be content.

Because in the current example we have only ID then we wrapping it with array but overridden methods can use that method by their own logic.