Well, thanks for looking at my source code!  At least at the top of the main .c file.  And yes, all strings with a suffix of “.onion” are automagically detected.  The bech32(1) tool does not know about any specific .onion sites, although I had to pick one for a manpage example.

If you like this particular feature, then you will be pleased to see the proposal I just made on tor-dev:

https://lists.torproject.org/pipermail/tor-dev/2017-December/012743.html