[Feature request] Add handler for source and layer loaded events

[markusand]

This utility library is really handy. One valuable utility that would solve many people's headaches would be to have a reliable way to detect when a source or layer has been loaded properly.

map.U.onSourceLoad('sourcename', features => {
    // Do whatever with features
});

map.U.onLayerLoad('layername', () => {

});