Peter M. Groen / CLI11

[skip ci]

Now use type_name and type_size instead of set_custom_option.

Clang++/LLVM 6.0 complains about this from warning
-Wdelete-non-virtual-dtor which is included when using -Wall.

Both App and FormatterBase already have virtual destructors, so this
was probably just accidentally left out.

This is also a C++ best practice; I have drawn some guidelines from
here: http://www.gotw.ca/publications/mill18.htm