Add method to swap registered generators

[dmason30]

Closes #236

Currently there is no way to override the default registered generators. This PR adds a swapGenerator method that was discussed in #236 with a matching test!

[jasonmccreary]

I'm going to merge this, but leave it undocumented until you've let me know it works as expected in your Pest Tests add-on.

[dmason30]

@jasonmccreary I have finished making the add-on https://github.com/fidum/laravel-blueprint-pestphp-addon it works great using the swapGenerator.

I haven't created a release for it yet as it is currently using laravel-shift/blueprint:dev-master. I will wait patiently for your next release with the new function included.

I did essentially copy parts of your TestGenerator and tweak it to suit my needs I hope that is acceptable, I will watch for any bug fixes and port them over.

Out of interest, would you consider adding an add-ons section to the docs site, I understand if not.

[jasonmccreary]

Excellent. I will tag a Blueprint release later this evening.

Once we confirm it in action and see some use, I'm glad to add it to the documentation and promote it via Twitter.