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, making it possible to use fields as direct properties (replacing the current `{{ post.get_field('my_custom_field') }}` syntax). Please advise if there are reasons unknown to me as to why this may be undesirable.