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 am still not sure if we should use a temp buffer instead of a ephemeral buffer