add woocommerce related product widget

[ivankristianto]

Changelogs:

1. Add new widget: EP_Related_Products_Widget
2. Add new function: ep_wc_get_related_products

Fixed #736

[tlovett1]

I'm experiencing some strange behavior:

I have three products with similar names. When looking at any of the products, they all show a related products which makes sense. When I add a product tag to any of them, none show in the widget.

What is the expected functionality for this? How does the algorithm work?

[ivankristianto]

The way it works is using ES more_like_this query:
https://www.elastic.co/guide/en/elasticsearch/reference/5.5/query-dsl-mlt-query.html

So here is the query in Related product widget:
https://github.com/10up/ElasticPress/blob/develop/features/related-posts/related-posts.php#L10

Where the more_like query is introduce in:
https://github.com/10up/ElasticPress/blob/develop/features/related-posts/related-posts.php#L10
Which I think we should move this to the ep_formatted_args to make it usable in WP_Query without activate Related posts feature.

[allan23]

Thank you for your interest in contributing to ElasticPress. At the time of this writing, we are focusing our development efforts on version 3.0 in the develop branch, which is an entirely different codebase. We ask that you please take some time to review the new codebase and alter your pull request as needed to accommodate these major changes.