Let local YCM config file override global config file

Currently, when VIM opens a source file, YCM always defaults to
'g:global_ycm_extra_conf_file' if it exists.

This commit changes YCM's behaviour so that it first tries to find the config
file in the source file's folder (or any of its parents folder), before
falling back to 'g:global_ycm_extra_conf_file'.