Allow theme users to hide the "Got something to add?" section

I don't use Twitter or Google+ so I fancy hiding the "Got something to add?" section. I would prefer to not maintain my own fork with this section commented out so thought I'd make its visibility configurable and see what you think.

As the commit message points out, it's a bit lame the Twitter and Google+ URLs are still hard-coded but that could be a job for another day (and probably another place).