Implemented unloadChildApplication api and unload lifecycle.
Features:
- Hot loading now possible for entire apps
waitForUnmountflag inunloadChildApplication, which tells single-spa to wait until it is "safe" to hot reload an application (when it's no longer active and has been unmounted).- Now possible to re-bootstrap applications, by first unloading them
Bug Fixes:
- Fixed bug where hashchange and popstate event listeners were getting called twice for the same HashChangeEvent or PopStateEvent