added tab shortcodes for tabbable content; added first-tab auto-activation; added generic content shortcode
changed "attachment_id" to "media_id" for clarity; added split-content shortcode for displaying content next to an image
added "media" option to split content shortcode to change order of content/image
added "icon content" shortcode for displaying an icon next to a paragraph or other content
Merge branch 'feature-shortcodes' into develop