Support for the apropos command in console

[MirceaKitsune]

As discussed on Discord, it would be nice to have an equivalent to Xonotic's (darkplaces engine) "apropos" command in the console. When using apropos word the console will list all settings containing word (foo_word / word_bar / foo_world_bar).

This is useful to find settings outside of the menu when doing tasks like development tweaks. For example, if you're looking for all settings related to player physics and know they begin with "g_player_physics_", you can use apropos player_physics to generate a list of each cvar + its value + its description (when available).