Make webthing-cpp WoT compatible

[bw-hro]

Add flavor option to generate interface that is compatible with WoT standard. This would allow to let the thing live in WoT ecosystem.

also cf.

• https://www.w3.org/TR/wot-thing-description
• https://playground.thingweb.io/
• http://plugfest.thingweb.io/webui/
• https://github.com/draggett/arena-webhub