Use @login_required next parameter

[cgwhitehead]

This is an awesome library that saved me a lot of time. After I implemented it, I was using the login_required decorator on my views, but noticed the next parameter was not working as it was not passed to the callback url and so the success redirect defaulted to the url in my settings.

##How this feature would work
In auth_helper.py it would check for a next parameter. If one exists, it would redirect to the next url. If no next parameter is passed, then you would redirect to the default success url.

##Customizing the lock
I just added a variable that appended location.search to the end of my callback url. Then the next parameter was available in the request.GET. I put an example on the readme in my fork.

##Working on my fork
I tried this on my fork, but I haven't built tests or checked for python versions other than 3.6.

I'm sorry if this breaks any protocol. I'm pretty new, and this would be the first time I've contributed.
Is this a feature that would benefit the project?