Merge ACF fields into post custom fields

Ideally, I would like to be able to use ACF custom fields directly in my page via `{{ post.my_custom_field }}` syntax ([which I've interpreted from the docs to be possible](https://github.com/jarednova/timber/wiki/TimberPost#wiki-properties)). This change automatically merges all ACF fields with a page's custom fields. Please advise if there are reasons unknown to me as to why this may be unadvisable.