Feature request: support cppman

[bchretien]

cppman downloads, caches and provides a way to get "C++ 98/11/14 manual pages for Linux/MacOS, with source from cplusplus.com and cppreference.com". Since cppman behaves like man, the implementation should not be that different from the current man support.

Note: cppman provides an option to add C++ manuals to mandb ($ cppman -m true), so another option could be to tell vim-ref users to use that option, and then use the current man support. In that case, some comments could be added to the doc.

[agauniyal]

+1