Use profile links from 'social' object

[aslafy-z]

While setting up this theme, I figured out that if I'm not using github social, 'site.github' where already defined by github itself. https://help.github.com/articles/repository-metadata-on-github-pages/#available-repository-metadata

I moved all social links to a 'social' object.

This PR comes in two chunks, one per branch.

[epidrome]

nice catch, thanks! I am reluctant to change so many variables in order to fix just one bug. For example the remote theme functionality will break for many existing users, who already have a config file with site.facebook instead of the proposed site.social.facebook.

Nevertheless, I have opened an issue #5 to consider the naming-loop of social icons separately and as a whole. The theme is still in alpha so we might want to introduce major updates as long as they offer significant benefits.

thanks again for reporting this bug!