Add menu for custom console variables

CV_RegisterVar({name = , defaultvalue = , flags = , PossibleValue = , func = }) -> CV_RegisterVar({name = , defaultvalue = , flags = , PossibleValue = , func = , category = , displayname = ,})

Added CV_MENU flag - adds console variable into the menu.

Name	Type	Accessibility	Description/Notes
displayname	string	Read-only	Name of console variable, currently only used in custom variable menu
category	string	Read-only	Category of the console variable, currently only used in custom variable menu

Both new variables are in consvar_t and vanilla console variables do not use them yet. They are currently NULL pointers.

cvar_menu_stress_test.lua