Load config and register providers

[kellymears]

Added two private methods to SageServiceProvider in order to load the Sage app configuration and register the providers from the providers array.

Addresses #14 .

[QWp6t]

ServiceProvider already has a mergeConfigFrom() method that can be used (sidenote: there's also loadViewsFrom()). I want there to be a single authority where a user can override any config files, and it needs to be configurable. Right now that's supposed to be <basepath>/config, and when Sage is found, the basepath is set to the theme folder. In short, we should use $this->mergeConfigFrom() as needed, but in this case, it's actually not needed since the config file is already getting loaded.

As for app providers not being loaded, that was a bug that I accidentally introduced a couple months ago when I pushed a bunch of updates. It should probably be handled either in a separate Bootstrap or within Application.php itself (or both). Feel free to refactor the PR if you want and I'll be happy to merge it. (note: this is how I used to handle it)

[kellymears]

It's straight up a privilege to have a chance to contribute to this awesome thing you've built.

As requested (let me know if I've misinterpreted anything):

Registration of configurable service providers is removed from Acorn\Sage\SageServiceProvider and added to Acorn\Application.

Adds a new bootstrap class to call this method: Acorn\Bootstrap\RegisterProviders. This bootstrap class has been added to the $bootstrap array in the bootloader() function of src/helpers.php.

With this change configurable service providers are successfully registered and booted based on the providers inventory in config/app.php.

[QWp6t]

Awesome dude! Thanks a bunch for helping with this. I haven't had a chance to revisit the Acorn stuff for a while, but I plan on getting to it sometime next week.