[0.3.3] - Null safety

Contributed by rkishan516, thanks. Updates the lib for null safety

[0.3.2] - Add optional click to active tab circle

Contributed by TheWithz, thanks. The circle can have a click event when active.

[0.3.1] - Fix wrong icon bug

Fixes bug where wrong icon is shown when navigating away and back

[0.3.0] - Remove Overlay widget causing huge problems

Removed Overlay widget
Added test
Changes to constructor, as RouteObserver is no longer needed.

[0.2.0] - Bug fixes, and new functionality

Breaking change!! Widget now requires a RouteObserver
Fixes bug where the active tabs circle is left on screen when navigating away
Adds key to the Widget to enable programmatic selection.

[0.1.1] - Small change to constructor

No longer required BuildContext to be passed.
Moves some init code outside initState

[0.1.0] - Initial Beta Release

Beta release