use cloud tiles as fallback

[goto-10]

related issue: #42 (comment)

changes:

• converts getWorldByName to async function
  • checks for a local map.json first, then falls back to the cloud cdn if enabled (ui.useCloudTiles)
  • if cloud is used _useCloudTiles property is injected into the world data (checked in initMap fn)
• refactored promise chain in processOp fn a bit to leverage the catch fn to show errors properly