add support for g:syntastic_ignore_files

Since we stopped using Syntastic for diagnostics display, this option
for Syntastic no longer works in YCM. So we should handle it ourselves.

Expaination of g:syntastic_ignore_files:

https://github.com/scrooloose/syntastic/blob/50518b335d6bc486c895d4cb34c29417238cbe81/doc/syntastic.txt#L384

P.S. I've signed the Google CLA.