Add option field for GitHub and include in bw_follow_me

[bobbingwide]

As for Twitter, Facebook etc, it would be nice if the [bw_follow_me] shortcode could display a link to the user's GitHub account. All it needs to store is the "owner" part, the rest of the GitHub url can be created.

Whether or not there needs to be a separate shortcode is debatable. If the [github] shortcode exists then we can probably use that? But what happens with the oik-user plugin.

An icon with the GitHub mark will also be needed for 'backward compatibility' when [bw_follow_me] is coded without theme=gener.

[bobbingwide]

This new logic should include test cases runnable under PHPUnit.

[bobbingwide]

• This change is tested in tests\test-oik-follow.php and test_oik_options_do_page
• The GitHub icon image used is images\github_48.png and images\github_old.png
• https://github.com/bobbingwide/oik/blob/master/images/github_48.png
• https://github.com/bobbingwide/oik/blob/master/images/github_old.png
• dashicons doesn't provide a GitHub icon, so it's not tested.
• The [github] shortcode is provided in https://github.com/bobbingwide/oik-bob-bing-wide
• The [github] shortcode is not designed to cater for oik-user