Allow usage of different Twitter and Github usernames

and show these links only if twitter-username or github.username are set on _config.yml
Default entries for twitter/github usernames
Change Github/Twitter keywords to site.{twitter,github}_username
Re: https://github.com/jekyll/jekyll/pull/2485/files#r13525347
Merge pull request #2485 from tomer/patch-2
Update history to reflect merge of #2485 [ci skip]