checks that the user wants c++

## motivation
In the future, Cap'n Proto should support many language bindings.  Also, I wanted a chance to read some of the code and get a feel for the structure of the tool.

## implementation
This lays the foundation for supporting many language bindings, by adding an input field for the user to ask for code to be generated for their language.

## disclaimer
I have no experience in haskell, and the code felt ugly while I was writing it, so I have a feeling that although it works, it doesn't do it the right way.  Most of my effort was put into ensuring that "Only c++ is supported for now" was printed only once, which should not have been a hard problem.  I would really appreciate some guidance on how to clean this up enough that it can be accepted into the project.