Add built-in functions for transformation

Users are forced to implement a custom mapper for trivial data manipulation tasks today (e.g. substring, trim, toLowerCase, etc.). It would be nice if we had a set of "built-in" functions that the user could select from within the UI instead of implementing a custom mapper. In terms of ideas for which built-in functions to start with, I would check out the methods on java.lang.String - many easy and useful methods to choose from.