Add github scope option to initializer.rb

[tackeyy]

As I found the undocumented option which is about Github's scope with external module in Github OAuth. I think it is better if this pull request is accept, because it takes time to find the option out if someone who uses Github OAuth with external module and want to manage the Github's scope.
I don't want to change original behavior when comment out of added option is remove, so default value is blank. (blank scope is called no scope .)

refs: #48

[joshbuker]

Seeing as this doesn't change the default value, I don't really see any problem adding this even without added documentation in the config file. I feel like setting up applications for all the different oauth providers isn't very well documented, so that's something that we should probably add to the wiki. (Links to the appropriate dev docs (scopes, available mappings, etc) and simple walkthrough for getting your app started on each provider.)

[tackeyy]

@athix thanks for approve.

I feel like setting up applications for all the different oauth providers isn't very well documented, so that's something that we should probably add to the wiki.

I agree. It's more useful if all (or popular) oauth providers is well documented. if I could updated github oauth providers document, I'll open other PR :)