[5.0] Added a config_path() helper function

[brunogaspar]

Added a config_path(:path) helper function, which i believe to make sense, it stays consistent with the app_path(:path) and base_path(:path) methods and will also help a bit when writing the Service Providers for the new publisher command.

Will you consider a database_path(:path) as well? I find both helper functions to be useful when writing the Service Providers as config and migrations are the most common things on packages, language files is another thing, but, i rarely see them.

[crynobone]

Not sure how useful config_path() as $path is not going to be used (as of current Laravel 5) with the way Illuminate\Config\Repository works right now.

[GrahamCampbell]

Yeh, @crynobone is correct.

[brunogaspar]

I found it useful when using the new package publish feature/command.

[GrahamCampbell]

The decision is ultimately up to Taylor. I'll leave this for him to review.

[taylorotwell]

Will add since it is somewhat useful for config publishing now.