Check for ACF and a specific ACF field and display if possible.

I've been using this setup to allow me to choose which category is shown when viewing a single post. ACF could be setup in various ways, but as long as the field is named ```primary_category```, and it's not a file field, this should work.

If ACF and the ```primary_category``` ACF field don't exist (ie: the plugin isn't enabled or hasn't been configured), we fall back to the original category display.

Thought others may like this. I'll have a blog post about my exact setup later on today.