extra_conf_globlist should use a preference

[psifertex]

Given that ycm-core/YouCompleteMe#1918 is apparently biting me despite having "ycmd.confirm_extra_conf": true set, it would be nice if extra_conf_globlist were also a user preference. I tested it locally and it worked fine, unfortunately the code in the released version looks pretty different from what's in the repo and I don't have the environment set up to build/test it, but the change is pretty trivial, just set it from preferences as opposed to the empty array.