Add code coverage via coveralls.io

- Add `TweenCompleted::user_data` to help identify the tween which
  raised the event.
- Improved several docs.
- Update CHANGELOG.
- Update README with mention of events and callbacks.

Bevy 0.6.1 was released which fixes nightly builds. Re-add nightly
toolchain to CI and remove the disclaimer from the README.

Bevy 0.6.0 is broken for nightly, which breaks all `docs.rs` builds.
Remove nightly CI and add a README warning until this is fixed in 0.6.1.

Export publicly the asset and component system to allow an app to
manually add them.

- Add `Sequence<T>` for chained tweens
- Add `Tracks<T>` for tracks of sequences running in parallel
- Move most animation-related properties to the new `Tweens<T>` struct
- Add `sequence` example

- Fix license name and color
- Add Bevy tracking badge

Seems like `bevy_easings` moved too... :)

Import `bevy_tweening` from its old repository.

URL:
https://github.com/djeedai/bevy_extra/tree/main/crates/bevy_tweening

Commit: 1e13a72