Allow steps to be limited to specific classes

[RhysLees]

This PR aims to allow steps to be limited to specific classes which allows you to setup steps for multiple models for example if you have steps for users and teams.

Example Usage:

// Defining User steps
Onboard::addStep('Limited User Step', User::class)
    ->link('/post/create');

// Defining Team steps
Onboard::addStep('Limited Team Step', Team::class)
    ->link('/post/create');

// Defining a step that is available to all classes
Onboard::addStep('Normal Step')
    ->link('/post/create');

The above will result in 1 step being available to all classes, and 2 steps being available to the User and Team classes:

Other classes will only see the Normal Step.
User classes will both see the Normal Step and Limited User Step.
Team classes will both see the Normal Step and Limited Team Step.

This should be a non breaking change as you are still able to define steps the same as before. Also, when you specify a class, it will merge the class steps and non-class specific steps as seen in the example above.

Solves: #8

[freekmurze]

Thanks!