Normalize locale format in getPluralIndex method

[joelstein]

The IETF standard for locales uses a hyphen (-) instead of an underscore (_).

This pull request makes it possible to translate strings (especially plurals) when the locale is something like en-US vs. en_US.

[shaedrich]

Not sure, if that would be the ideal place. I think about this more generally like a config option where you can switch from POSIX to IETF BCP

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