implement white-/blacklisting

Here is my pull request for white-/blacklisting as discussed in PR #148.

Extract from `README.md`:

> The `g:ycm_extra_conf_globlist` option
> 
> This option is a list that may contain several globbing patterns.  If a pattern starts with a '!' all `.ycm_extra_conf.py` files matching that pattern will be blacklisted, that is they won't be loaded and no confirmation dialog will be shown.  If a pattern does not start with a '!' all files matching that patter will be whitelisted.  Note that items earlier in the list will take precedence over those later in the list.