Allow extensions to run core WC cart handlers

Rather than determining how to handle a product being added to the cart
based on its type, create a handler flag which extensions can set in
order to run core WC handlers for different product types.
Whitespace tweak
Merge pull request #2539 from thenbrent/master

Allow extensions to run core WC add-to-cart handlers
filter is_enabled boolean Closes #2535.
Stock increase/descrease alerts Closes #2536.
order terms by parent Closes #2534.