Unable to get state value on cell.getValuePrepareFunction callback

It would be nice to be able to get some additional cell state information on the cell.getValuePrepareFunction callback, this would allow round tripping optional information state settings on the cell, column or row of that cell.

**proposed change to**
https://github.com/akveo/ng2-smart-table/blob/master/src/ng2-smart-table/lib/data-set/cell.ts#L17

**from**
return prepare.call(null, this.value);
**to**
return prepare.call(null, this.value, {row: this.row, column: this.column, dataSet: this.dataSet});

that would allow access to some additional cell state without breaking existing code

**reason for request**
I am interested in being able to store state information on the cell settings and would like to retrieve that information on the callback