Make CMAKEPP_LANG_DEBUG_MODE an explicit CMake option

[zachcran]

Is your feature request related to a problem? Please describe.
CMAKEPP_LANG_DEBUG_MODE is an option that can be turned on to get better logging and error checks (although they are computationally more expensive to do). While this is available in the code and documented in almost every function, it is not advertised as a CMake option.

Describe the solution you'd like
Explicitly add it as a CMake option. This would probably be a single call in the top-level CMakeLists.txt:

option(
    CMAKEPP_LANG_DEBUG_MODE
    "Enable CMakePP's debug mode. This is computationally expensive and should be left off unless checking for programming errors."
    OFF
)

Describe alternatives you've considered
Leave it the way it is, as a "hidden" option.