Allow usage of different Twitter and Github usernames in default theme

Currently it is impossible to use different names on Github and Twitter without changing the default template files. I suggest using different keywords in order to make it easier to customize the output without digging into the theme files. 

As a added-value, I also added conditional to show these links only if the keywords are set on _config.yml, so people who would like to show only their twitter identity should just remove the github-username keyword from the config file. ☺
Let's use underscores here instead of hyphens: `twitter_username` and `github_username`, respectively.