1. Make sure all the changes you made to your config are final so you ain't gonna edit it.
   
2. Create a direct copy of it and name it however you want, e.g. gaben is love gaben is life.cfg (ALWAYS .cfg).
   
3. Create autoexec.cfg and inside it put:
   
   exec gaben is love gaben is life.cfg
4. ???
   
5. PROFIT!

Since 99,99% of players don't back up their configs, playing on some kind of freaked out server with Tank+horde setting and shadows as dark as Louis' butth♥le results in need of veryfing game cache or, more advanced, deleting config.cfg file, which anyway results in getting default config. This way you need only to restart your game since edits are made to config.cfg file, which are discarded after starting L4D again cause any changes are overwritten anyway by the config executed by autoexec.cfg.

Don't forget to check the other guide from serie „How to save yourself 10 years of lifetime by not getting mad about modded Russian servers”:

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=541741732