Add support for optional custom fixers and rules in pint.json

[mho22]

Based on PR suggestion : #26

It allows to add some external rules into your current projet, if needed :

{
    "fixers": [
        "External\\Custom\\Fixer"
    ],
    "rules": {
        "External/Custom/Fixer/rule": {
            "foo": true
        }
    }
}

This doesn't break the actual use of Laravel Pint, it opens the possibility to use the custom fixers from Pint outside of it.

P.S.: Some test files have been reorganized due to the actual use of fixers directory.

[mho22]

Apologies for the multiple unnecessary commits. I encountered difficulties meeting the checks.

By the way, Merry Christmas! 🎄

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If possible, please consider releasing your code as a package so that the community can still take advantage of your contributions!

If you feel absolutely certain that this code corrects a bug in the framework, please "@" mention me in a follow-up comment with further explanation so that GitHub will send me a notification of your response.