Create a URLExtractorModelLoader

[andrus]

create URLExtractorModelLoader class. It should supersede a very limiting ClasspathExtractorModelLoader and will make integration with Bootique easier (e.g. bootique/bootique-linkmove#5 would become trivial). Add the following method to LmRuntimeBuilder that will use the new class internally:

LmRuntimeBuilder extractorModelsRoot(URL rootUrl) { ..}