Implement New WebKit Status: Supported In Preview.

[futursolo]

Recently, WebKit has changed its feature life cycle and added a new feature status called Supported in Preview; however, FireFox platform status treats this status as No Signal.

This can be fixed by adding a switch case in https://github.com/mozilla/platform-status/blob/master/engine/index.js#L26.

Also, under the current mechanism, all the WebKit features show as Safari Technology Preview features. It would be better if the features in Supported status show as Safari features and features in Supported in Preview status show as Safari Technology Preview features.

References:
Web Cryptography API in https://svn.webkit.org/repository/webkit/trunk/Source/WebCore/features.json
WebAssembly in https://svn.webkit.org/repository/webkit/trunk/Source/JavaScriptCore/features.json

[marco-c]

I'm not sure what we should do to be honest. I don't know if all the "Supported" features on webkit.org/status are actually supported by Safari or only in Preview.

So, for now with #481 we will consider "Supported in Preview" as "In Development", which is better than nothing.