company-ycmd: Fontify meta string in minibuffer

Use font-lock for meta string in minibuffer

Inspired by `ggtags-fontify-code` function from `ggtags.el`: https://github.com/leoliu/ggtags/blob/master/ggtags.el#L1965-L1982
I feel more comfortable with `with-temp-buffer`, but only because I don't see a reason to keep the buffer around. I'll merge this version, and we can fix it later if we see any problems.