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]
Run Rubocop on our codebase.
Add Rubocop to help maintain code style
Fixing use of deprecated class methods

Updates File.exists? to File.exist?
Set rubocop to ensure end alignment with variable
Fixing block variable shadowing outer local variable
Prepending underscore to unused block variables
Disable checking of unused method arguments
Using && and || in place of styleguide 'banned' words
Removes trailing whitespace
Remove trailing comma
Adding newline to file
Fix space inside square brackets