Changes in CDWKWebViewEngine to reflect changes in CDVWebViewEngineProtocol.

Made loadHTMLString and loadRequest explicit - loadRequest implicitly handles file urls by attempting to use loadFileURL if available.
Added CordovaLocalWebServerStartOnSimulator setting, which defaults to true.

If set to false, when the user is running on the Simulator, we don't start the local web server. This is primarily to facilitate testing the wkwebview-engine plugin, which has a bug where file urls don't load on device (which is by design, there is a new loadFileURL API function, unreleased at this time) --  but file urls do load on Simulator.
Renamed SACordovaLocalWebServer -> CDVLocalWebServer
Plugin wasn't loading due to renamed iso-package name.
Updated wkwebview-engine README
The local web server will load the app under the /app/ path, not the root. This is to facilitate other handlers, and not conflict with names of the pages in the app contents.
Renamed app path from 'app' to 'www'.
Added preliminary /asset-library/ and /local-filesystem/ support, incomplete.