Issue 612 - Custom connectors contexts removal from post/taxonomy connectors

@fjarrett Please review. And if you can please test woocommerce integration, might need some more work, as I've made it so related post-types/taxonomies records are registered under its connector name.

Resolves #612 
@shadyvb Hmmm. It seems like logging payments would be a crucial metric for Stream to track, especially considering setting up notifications with payment amounts as meta and such. The payments post type itself is not a log, so I'm not sure I understand what you mean here.