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
Could we use `with-temporary-buffer` here instead? I don't see a reason to keep the buffer around after we return its contents, unless there's some performance issue.