ENH: add new help option --help-custom-modules, which generates
documentation for all modules found in CMAKE_MODULE_PATH, which currently
has to be specified via -D, this can later on be improved e.g. by reading a
special (to-be-created) file like CMakeFiles/ModulePath.cmake in the build
tree so that running cmake help in the build tree of a project will always
give you the current module path. (This could actually also help IDEs which
would like to support cmake for projects...)

Alex