Filter in new settings for store address, address_2, city and postcode

[allendav]

Fixes #12

See also woocommerce/woocommerce#15636

To test:

• Ensure GET wp-json/wc/v2/settings/general returns the new settings
• Ensure POSTing to wp-json/wc/v2/settings/general/{setting} stores the passed value for each of the following

woocommerce_store_address
woocommerce_store_address_2
woocommerce_store_city
woocommerce_store_postcode

• Ensure if this plugin is active at the same time as Add store street address, city and postal code to settings woocommerce#15636 the the world doesn't asplode and that the GET response only returns each of the new settings once

[emdashcodes]

LGTM and thanks for the comment/notice to make it clear this is temporary.

Followed all testing directions and everything works, including with the WC branch applied.