[Discussion] Redesigning script for generating new exercises

@exercism/dart @devkabiir and I have been talking about how to improve the script for generating exercises in #170.

A few points:
* @devkabiir brought up the idea of using a templating engine like mustache
* Whatever we do, it needs to be flexible as there are some relational aspects to the generated code and the input that could be complex.

List of cases that need to be considered:
* A way to inform the generator whether to place the reference the function calls in test suite within a class instance or not. Would also inform the generator whether to create a class in the empty exercise file in `lib`