Support for Sonoff TH Elite 20A and TH Origin 20A

[jonathanschneider]

This PR adds support for Sonoff TH Elite 20A and TH Origin 20A which use a double-coil latching relay. The pins to control the relay are hardcoded since they're defined by these devices and cannot be changed. A more elegant solution could be to introduce new pin types such as "Latching SET" and "Latching RESET" but since I haven't found any requests for latching relays it's probably not worth the effort.

[jonathanschneider]

This PR is based on @zandegran's PRs #438 and #440.