Fixes undefined TargetPosition by exporting /src/target/target_position.dart

[vemarav]

export 'package:tutorial_coach_mark/src/target/target_position.dart';

closes #56

[EducatedDeer]

???
still can't use the TargetPosition on the newer version

[vemarav]

@EducatedDeer you can use git repo as package directly

tutorial_coach_mark:
  git:
    url: https://github.com/vemarav/tutorial_coach_mark

fork it and replace url until this fix lands on next version

[RafaelBarbosatec]

Thanks for contribution!