Consider adding a generalized transformation directive

[tzlaine]

From #119:

Maybe something for the future: there's a lot of effort in the code to get no_case working, and in
the end it's all about applying case folding. What about having a transformation-directive letting
users plug-in their own transformations for input and target:

transform( _input_transformation_ )[ parser ]
transform( _input_transformation_, _target_transformation_ )[ parser ]

[tzlaine]

This is very low priority. I like the idea, but it's currently searching for a use case. I'm leaving it open so that 1) it is not forgotten, and 2) others who do have a use case for this might stumble upon it.