Add function addCharAtInterval() to Str class

[Ashraam]

This pull request adds a new function, addCharAtInterval, to the Str class.
This function adds a character to the subject every X occurrences.

Example:
Str::addCharAtInterval('123456789', '-', 3) will return 123-456-789

[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!