[6.x] Add wherePivotNotIn and orWherePivotNotIn methods

[webchopin]

The main goal of this PR is to add more convenience.

Similar to what is done in Illuminate\Database\Query\Builder with whereNotIn and orWhereNotIn, I added 2 methods called wherePivotNotIn and orWherePivotNotIn.
I did it in exactly the same way whereNotIn and orWhereNotIn are built.

Instead of a call like:

$post->wherePivotIn('flag',['foo','bar'],'and',true);

with this PR, developers could use

$post->wherePivotNotIn('flag',['foo','bar']);

Basically, $post->wherePivotIn('flag',['foo','bar'],'and',true); will be called behind the scenes.

I also added tests for both methods. Besides, since I could not find any test for the orWherePivotIn method which was already part of the framework before this PR, I added a test for this method also.

Based on the contribution guide, I assumed it was a minor feature fully backward compatible since the added methods are new Laravel API methods which are not referenced anywhere else in the framework. That's why I pushed it to the latest stable branch. If you prefer to have it on the master branch, please let me know.

Also, if the PR is accepted, please let me know if you want to mention wherePivotNotIn in the documentation. I couldn't find orWherePivotIn in the documentation, so I guess there is no need to add orWherePivotNotIn either.