Create 'tf/cfg/common.cfg'.

Copying the concept of a user-written hook for common input the
configuration file 'common.cfg' sets several CVars that a user might toy
with during a game or change locally for a class. The script is executed
after 'config_default' but before 'config/common'.

Such a file is a move isolating classes futher.

Currently, the script contains CVar values to prevent downloading custom
content, enabling raw input, and increasing the field of view.