Consider adding an option to add rel="nofollow" to the <a> tag

Decide whether to add nofollow as a default, or add an option on the settings page to toggle it on/off.

Locations where nofollow should be added:

1. https://github.com/rumspeed/simple-side-tab/blob/release-v1.2.0/simple_side_tab.php#L224
2. https://github.com/rumspeed/simple-side-tab/blob/release-v1.2.0/simple_side_tab.php#L229