Loading `.ycm_extra_conf.py`: Potential security issue

I thought about the automatic loading of `.ycm_extra_conf.py` from any parent directory. Since I use vim to inspect source code one would only need to place such a file in a archive/repo and as soon as I look at a file any code contained therein would be executed (delete all files, install keylogger, …).

Maybe an option to use a prompt / confirmation is necessary (see patch). Something like:
```
Found /…/some_repo/.ycm_extra_conf.py. Load?
[Y]es, (N)o:
```
One other possibility would be to enable/disable loading of `.ycm_extra_conf.py` for certain paths (e.g. exclude ~/Download).

**This pull request is only to get the discussion started** (as it lacks documentation and I'm not sure if I formatted as you intended).