async initialisation

[dcooley]

will be part of 3.0.0 release

• [x] use dynamic bootstrap loader
  • [x] load marker library
  • [x] load places library
  • [x] load visualization library
  • [x] load drawing library
  • [ ] load user-defined libraries
• [ ] add idle listener so users can hook-into this callback once the map is loaded.
• [ ] Needs to work for more than one widget
• [x] async Markers
  • [x] Use AdvancedMarkerElement (#270)
• [ ] add_kml
• [ ] info windows
• [ ] mouseover / mouseenter / mouseleave